Data transmission method in wireless communication system and device therefor

ABSTRACT

Disclosed is an uplink (UL) multi-user (MU) transmission method by a station (STA) in a wireless LAN (WLAN) system. The uplink multi-user transmission method by an STA in a WLAN system according to the present invention includes the steps of: receiving a trigger frame including resource unit allocation information for orthogonal frequency division multiple access (OFDMA) transmission; transmitting a UL MU physical protocol data unit (PPDU) frame, on the basis of frequency resource allocation information; and receiving an ACK frame for the UL MU PPDU.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2015/012917, filed on Nov. 30, 2015,which claims the benefit of U.S. Provisional Application No. 62/087,809,filed on Dec. 5, 2014, 62/089,243, filed on Dec. 9, 2014 and 62/109,622,filed on Jan. 30, 2015, the contents of which are all herebyincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method for performing or supporting uplinksingle user/multi-user transmission and an apparatus that supports thesame.

BACKGROUND ART

Wi-Fi is a wireless local area network (WLAN) technology which enables adevice to access the Internet in a frequency band of 2.4 GHz, 5 GHz or60 GHz.

A WLAN is based on the institute of electrical and electronic engineers(IEEE) 802.11 standard. The wireless next generation standing committee(WNG SC) of IEEE 802.11 is an ad-hoc committee which is worried aboutthe next-generation wireless local area network (WLAN) in the medium tolonger term.

IEEE 802.11n has an object of increasing the speed and reliability of anetwork and extending the coverage of a wireless network. Morespecifically, IEEE 802.11n supports a high throughput (HT) providing amaximum data rate of 600 Mbps. Furthermore, in order to minimize atransfer error and to optimize a data rate, IEEE 802.11n is based on amultiple inputs and multiple outputs (MIMO) technology in which multipleantennas are used at both ends of a transmission unit and a receptionunit.

As the spread of a WLAN is activated and applications using the WLAN arediversified, in the next-generation WLAN system supporting a very highthroughput (VHT), IEEE 802.11ac has been newly enacted as the nextversion of an IEEE 802.11n WLAN system. IEEE 802.11ac supports a datarate of 1 Gbps or more through 80 MHz bandwidth transmission and/orhigher bandwidth transmission (e.g., 160 MHz), and chiefly operates in a5 GHz band.

Recently, a need for a new WLAN system for supporting a higherthroughput than a data rate supported by IEEE 802.11ac comes to thefore.

The scope of IEEE 802.11ax chiefly discussed in the next-generation WLANtask group called a so-called IEEE 802.11ax or high efficiency (HEW)WLAN includes 1) the improvement of an 802.11 physical (PHY) layer andmedium access control (MAC) layer in bands of 2.4 GHz, 5 GHz, etc., 2)the improvement of spectrum efficiency and area throughput, 3) theimprovement of performance in actual indoor and outdoor environments,such as an environment in which an interference source is present, adense heterogeneous network environment, and an environment in which ahigh user load is present and so on.

A scenario chiefly taken into consideration in IEEE 802.11ax is a denseenvironment in which many access points (APs) and many stations (STAs)are present. In IEEE 802.11ax, the improvement of spectrum efficiencyand area throughput is discussed in such a situation. More specifically,there is an interest in the improvement of substantial performance inoutdoor environments not greatly taken into consideration in existingWLANs in addition to indoor environments.

In IEEE 802.11ax, there is a great interest in scenarios, such aswireless offices, smart homes, stadiums, hotspots, andbuildings/apartments. The improvement of system performance in a denseenvironment in which many APs and many STAs are present is discussedbased on the corresponding scenarios.

In the future, it is expected in IEEE 802.11ax that the improvement ofsystem performance in an overlapping basic service set (OBSS)environment, the improvement of an outdoor environment, cellularoffloading, and so on rather than single link performance improvement ina single basic service set (BSS) will be actively discussed. Thedirectivity of such IEEE 802.11ax means that the next-generation WLANwill have a technical scope gradually similar to that of mobilecommunication. Recently, when considering a situation in which mobilecommunication and a WLAN technology are discussed together in smallcells and direct-to-direct (D2D) communication coverage, it is expectedthat the technological and business convergence of the next-generationWLAN based on IEEE 802.11ax and mobile communication will be furtheractivated.

DISCLOSURE Technical Problem

An embodiment of the present invention provides an uplink single user ormulti-user transmitting method in a wireless communication system.

An embodiment of the present invention further provides an uplink framestructure for supporting uplink single user or multi-user transmissionin a wireless communication system.

Technical objects to be achieved by the present invention are notlimited to the aforementioned object, and those skilled in the art towhich the present invention pertains may evidently understand othertechnical objects from the following description.

Technical Solution

In an aspect of the present invention, an uplink (UL) multi-user (MU)transmitting method of a station (STA) in a wireless LAN (WLAN) systemincludes: receiving a trigger frame including resource unit allocationinformation for orthogonal frequency division multiple access (OFDMA)transmission; transmitting a UL MU Physical Protocol Data Unit (PPDU)based on the trigger frame; and receiving an ACK frame of the UL MUPPDU, wherein the trigger frame includes a first legacy preamble and afirst High Efficiency (HE) preamble, and the first HE preamble includesa first HE-SIG-A field and a first HE-SIG-B field, and the firstHE-SIG-A field includes first Transmission Opportunity (TXOP) durationinformation representing a first TXOP duration, and the first TXOPduration is the remaining time interval of a frame exchange sequence ofthe STA.

The first TXOP duration may include a length in time of the UL MU PPDUand a length in time of the ACK frame.

The UL MU PPDU may include a second legacy preamble and a second HEpreamble, and the second HE preamble may include a second HE-SIG-Afield, the second HE-SIG-A field may include second TXOP durationinformation representing a second TXOP duration, and the second TXOPduration may be the remaining time interval of a frame exchange sequenceof the STA.

The second TXOP duration may include a length in time of the ACK frame.

The first TXOP duration may further include an Inter Frame Space (IFS)time between the trigger frame and the UL MU frame and an IFS timebetween the UL MU frame and the ACK frame.

The second TXOP duration may further include an IFS time between the ULMU PPDU and the ACK frame.

The second HE preamble of the UL MU PPDU may include a HighEfficiency-Short Training Field (HE-STF), a HE-Long Training Field(HE-LTF), and a data field, and the HE-STF, the HE-LTF, and the datafield may be transmitted through a bandwidth of an allocated resourceunit.

In another aspect of the present invention, a station (STA) apparatus ofa wireless LAN (WLAN) system includes: a Radio Frequency (RF) unit thattransmits and receives a wireless signal; and a processor that controlsthe RF unit, wherein the STA apparatus receives a trigger frameincluding resource unit allocation information for orthogonal frequencydivision multiple access (OFDMA) transmission, transmits a UL MUPhysical Protocol Data Unit (PPDU) based on the trigger frame, andreceives an ACK frame of the UL MU PPDU, the trigger frame includes afirst legacy preamble and a first High Efficiency (HE) preamble, and thefirst HE preamble includes a first HE-SIG-A field and a first HE-SIG-Bfield, and the first HE-SIG-A field includes first TransmissionOpportunity (TXOP) duration information representing a first TXOPduration, and the first TXOP duration is the remaining time interval ofa frame exchange sequence of the STA apparatus.

In another aspect of the present invention, an uplink (UL) multi-user(MU) receiving method of an Access Point (AP)-Station (STA) in awireless LAN (WLAN) system includes:

transmitting a trigger frame including resource unit allocationinformation for orthogonal frequency division multiple access (OFDMA)transmission; receiving a UL MU Physical Protocol Data Unit (PPDU) basedon the trigger frame; and transmitting an ACK frame of the UL MU PPDU,wherein the trigger frame includes a first legacy preamble and a firstHigh Efficiency (HE) preamble, and the first HE preamble includes afirst HE-SIG-A field and a first HE-SIG-B field, and the first HE-SIG-Afield includes first Transmission Opportunity (TXOP) durationinformation representing a first TXOP duration, and the first TXOPduration is the remaining time interval of a frame exchange sequence ofthe STA.

Advantageous Effects

According to an embodiment of the present invention, in a wirelesscommunication system, a plurality of users can smoothly performmulti-user transmission through an independent resource.

Further, according to an embodiment of the present invention, a wirelesscommunication system can support transmission of an uplink multi-user ina unit of a resource unit.

According to an embodiment of the present invention, TXOP protection ofan UL MU procedure can be effectively performed. In other words,according to an embodiment of the present invention, by including a TXOPduration field in a HE-SIG-A field, an STA, having received only atrigger frame within a BSS and the STA that overhears only the triggerframe in an OBSS can set a NAV. According to an embodiment of thepresent invention, by including a TXOP duration field in a trigger frameand an UL MU frame, STAs that recognize only one of the trigger frameand the UL MU frame can set a NAV through a TXOP duration field of theUL MU frame.

By including a TXOP duration field in a HE-SIG-A field, while deviatingfrom a capacity limitation of an L-SIG field, an erroneous operation oflegacy STAs can be prevented. Further, even when other STAs do notdecode a MAC header of frames transmitted and received in an UL MUprocedure, by decoding fields up to the HE-SIG-A field, the other STAscan set a NAV.

Advantages which may be obtained in the present invention are notlimited to the aforementioned advantages, and various other advantagesmay be evidently understood by those skilled in the art to which thepresent invention pertains from the following description.

DESCRIPTION OF DRAWINGS

The accompany drawings, which are included to provide a furtherunderstanding of the present invention and are incorporated on andconstitute a part of this specification illustrate embodiments of thepresent invention and together with the description serve to explain theprinciples of the present invention.

FIG. 1 is a diagram showing an example of an IEEE 802.11 system to whichan embodiment of the present invention may be applied.

FIG. 2 is a diagram illustrating the structure of layer architecture inan IEEE 802.11 system to which an embodiment of the present inventionmay be applied.

FIG. 3 illustrates a non-HT format PPDU and HT format PPDU in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

FIG. 4 illustrates a VHT format PPDU format in a wireless communicationsystem to which an embodiment of the present invention may be applied.

FIG. 5 is a diagram illustrating a constellation for classifying theformats of PPDUs in a wireless communication system to which anembodiment of the present invention may be applied.

FIG. 6 illustrates the format of an MAC frame of an IEEE 802.11 systemto which an embodiment of the present invention may be applied.

FIG. 7 is a diagram illustrating a frame control field within a MACframe in a wireless communication system to which an embodiment of thepresent invention may be applied.

FIG. 8 illustrates a VHT format of an HT control field in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

FIG. 9 is a diagram for illustrating a random backoff period and a frametransmission procedure in a wireless communication system to which anembodiment of the present invention may be applied.

FIG. 10 is a diagram illustrating an IFS relation in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

FIG. 11 is a diagram illustrating a DL MU PPDU format in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

FIG. 12 is a diagram illustrating a DL MU PPDU format in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

FIG. 13 is a diagram illustrating a DL MU-MIMO transmission process in awireless communication system to which an embodiment of the presentinvention may be applied.

FIG. 14 is a diagram illustrating an ACK frame in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

FIG. 15 is a diagram illustrating a block ACK request frame in awireless communication system to which an embodiment of the presentinvention may be applied.

FIG. 16 is a diagram illustrating the BAR information field of a blockACK request frame in a wireless communication system to which anembodiment of the present invention may be applied.

FIG. 17 is a diagram illustrating a block ACK frame in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

FIG. 18 is a diagram illustrating the BA Information field of a blockACK frame in a wireless communication system to which an embodiment ofthe present invention may be applied.

FIG. 19 is a diagram illustrating a High Efficiency (HE) format PPDUaccording to an embodiment of the present invention.

FIG. 20 is a diagram illustrating a HE format PPDU according to anembodiment of the present invention.

FIG. 21 is a diagram illustrating a HE format PPDU according to anembodiment of the present invention.

FIG. 22 is a diagram illustrating a HE format PPDU according to anembodiment of the present invention.

FIG. 23 is a diagram illustrating an uplink multi-user transmittingprocedure according to an embodiment of the present invention.

FIG. 24 is a diagram illustrating UL MU transmission according to anembodiment of the present invention.

FIG. 25 is a diagram illustrating UL MU transmission according to anembodiment of the present invention.

FIG. 26 is a diagram illustrating a HE frame according to an embodimentof the present invention.

FIG. 27 is a diagram illustrating a UL MU procedure and TXOP protectionaccording to an embodiment of the present invention.

FIG. 28 is a diagram illustrating a UL MU procedure and TXOP protectionaccording to an embodiment of the present invention and particularlyillustrates an example of a cascade structure in which a UL MU procedureis transmitted together with a DL MU frame.

FIG. 29 is a diagram illustrating a UL MU procedure and TXOP protectionaccording to an embodiment of the present invention and illustrates anexample of a cascade structure in which a UL MU procedure is transmittedtogether with a DL MU frame.

FIG. 30 is a diagram illustrating a UL MU frame and TXOP protectionaccording to an embodiment of the present invention.

FIG. 31 is a diagram illustrating an STA apparatus according to anembodiment of the present invention.

FIG. 32 is a diagram illustrating a method of transmitting/receiving aUL MU according to an embodiment of the present invention.

MODE FOR INVENTION

Hereinafter, some embodiments of the present invention are described indetail with reference to the accompanying drawings. The detaileddescription to be disclosed herein along with the accompanying drawingsis provided to describe exemplary embodiments of the present inventionand is not intended to describe a sole embodiment in which the presentinvention may be implemented. The following detailed descriptionincludes detailed contents in order to provide complete understanding ofthe present invention. However, those skilled in the art will appreciatethat the present invention may be implemented even without such detailedcontents.

In some cases, in order to avoid making the concept of the presentinvention vague, the known structure and/or device may be omitted or maybe illustrated in the form of a block diagram based on the core functionof each structure and/or device.

Furthermore, specific terms used in the following description areprovided to help understanding of the present invention, and suchspecific terms may be changed into other forms without departing fromthe technological spirit of the present invention.

The following technologies may be used in a variety of wirelesscommunication systems, such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), and non-orthogonalmultiple access (NOMA). CDMA may be implemented using a radiotechnology, such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be implemented using a radio technology, such asglobal system for Mobile communications (GSM)/general packet radioservice (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMA maybe implemented using a radio technology, such as institute of electricaland electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX),IEEE 802.20, or evolved UTRA (E-UTRA). UTRA is part of a universalmobile telecommunications system (UMTS). 3^(rd) generation partnershipproject (3GPP) long term evolution (LTE) is part of an evolved UMTS(E-UMTS) using evolved UMTS terrestrial radio access (E-UTRA), and itadopts OFDMA in downlink and adopts SC-FDMA in uplink. LTE-advanced(LTE-A) is the evolution of 3GPP LTE.

Embodiments of the present invention may be supported by the standarddocuments disclosed in at least one of IEEE 802, 3GPP, and 3GPP2, thatis, radio access systems. That is, steps or portions that belong to theembodiments of the present invention and that are not described in orderto clearly expose the technical spirit of the present invention may besupported by the documents. Furthermore, all terms disclosed in thisdocument may be described by the standard documents.

In order to more clarify a description, 3GPP LTE/LTE-A is chieflydescribed, but the technical characteristics of the present inventionare not limited thereto.

System to which the Present Invention can be Applied

FIG. 1 is a diagram showing an example of an IEEE 802.11 system to whichan embodiment of the present invention may be applied.

The IEEE 802.11 configuration may include a plurality of elements. Theremay be provided a wireless communication system supporting transparentstation (STA) mobility for a higher layer through an interaction betweenthe elements. A basic service set (BSS) may correspond to a basicconfiguration block in an IEEE 802.11 system.

FIG. 1 illustrates that three BSSs BSS 1 to BSS 3 are present and twoSTAs (e.g., an STA 1 and an STA 2 are included in the BSS 1, an STA 3and an STA 4 are included in the BSS 2, and an STA 5 and an STA 6 areincluded in the BSS 3) are included as the members of each BSS.

In FIG. 1, an ellipse indicative of a BSS may be interpreted as beingindicative of a coverage area in which STAs included in thecorresponding BSS maintain communication. Such an area may be called abasic service area (BSA). When an STA moves outside the BSA, it isunable to directly communicate with other STAs within the correspondingBSA.

In the IEEE 802.11 system, the most basic type of a BSS is anindependent a BSS (IBSS). For example, an IBSS may have a minimum formincluding only two STAs. Furthermore, the BSS 3 of FIG. 1 which is thesimplest form and from which other elements have been omitted maycorrespond to a representative example of the IBSS. Such a configurationmay be possible if STAs can directly communicate with each other.Furthermore, a LAN of such a form is not previously planned andconfigured, but may be configured when it is necessary. This may also becalled an ad-hoc network.

When an STA is powered off or on or an STA enters into or exits from aBSS area, the membership of the STA in the BSS may be dynamicallychanged. In order to become a member of a BSS, an STA may join the BSSusing a synchronization process. In order to access all of services in aBSS-based configuration, an STA needs to be associated with the BSS.Such association may be dynamically configured, and may include the useof a distribution system service (DSS).

In an 802.11 system, the distance of a direct STA-to-STA may beconstrained by physical layer (PHY) performance. In any case, the limitof such a distance may be sufficient, but communication between STAs ina longer distance may be required, if necessary. In order to supportextended coverage, a distribution system (DS) may be configured.

The DS means a configuration in which BSSs are interconnected. Morespecifically, a BSS may be present as an element of an extended form ofa network including a plurality of BSSs instead of an independent BSS asin FIG. 1.

The DS is a logical concept and may be specified by the characteristicsof a distribution system medium (DSM). In the IEEE 802.11 standard, awireless medium (WM) and a distribution system medium (DSM) arelogically divided. Each logical medium is used for a different purposeand used by a different element. In the definition of the IEEE 802.11standard, such media are not limited to the same one and are also notlimited to different ones. The flexibility of the configuration (i.e., aDS configuration or another network configuration) of an IEEE 802.11system may be described in that a plurality of media is logicallydifferent as described above. That is, an IEEE 802.11 systemconfiguration may be implemented in various ways, and a correspondingsystem configuration may be independently specified by the physicalcharacteristics of each implementation example.

The DS can support a mobile device by providing the seamless integrationof a plurality of BSSs and providing logical services required to handlean address to a destination.

An AP means an entity which enables access to a DS through a WM withrespect to associated STAs and has the STA functionality. The movementof data between a BSS and the DS can be performed through an AP. Forexample, each of the STA 2 and the STA 3 of FIG. 1 has the functionalityof an STA and provides a function which enables associated STAs (e.g.,the STA 1 and the STA 4) to access the DS. Furthermore, all of APsbasically correspond to an STA, and thus all of the APs are entitiescapable of being addressed. An address used by an AP for communicationon a WM and an address used by an AP for communication on a DSM may notneed to be necessarily the same.

Data transmitted from one of STAs, associated with an AP, to the STAaddress of the AP may be always received by an uncontrolled port andprocessed by an IEEE 802.1X port access entity. Furthermore, when acontrolled port is authenticated, transmission data (or frame) may bedelivered to a DS.

A wireless network having an arbitrary size and complexity may include aDS and BSSs. In an IEEE 802.11 system, a network of such a method iscalled an extended service set (ESS) network. The ESS may correspond toa set of BSSs connected to a single DS. However, the ESS does notinclude a DS. The ESS network is characterized in that it looks like anIBSS network in a logical link control (LLC) layer. STAs included in theESS may communicate with each other. Mobile STAs may move from one BSSto the other BSS (within the same ESS) in a manner transparent to theLLC layer.

In an IEEE 802.11 system, the relative physical positions of BSSs inFIG. 1 are not assumed, and the following forms are all possible.

More specifically, BSSs may partially overlap, which is a form commonlyused to provide consecutive coverage. Furthermore, BSSs may not bephysically connected, and logically there is no limit to the distancebetween BSSs. Furthermore, BSSs may be placed in the same positionphysically and may be used to provide redundancy. Furthermore, one (orone or more) IBSS or ESS networks may be physically present in the samespace as one or more ESS networks. This may correspond to an ESS networkform if an ad-hoc network operates at the position in which an ESSnetwork is present, if IEEE 802.11 networks that physically overlap areconfigured by different organizations, or if two or more differentaccess and security policies are required at the same position.

In a WLAN system, an STA is an apparatus operating in accordance withthe medium access control (MAC)/PHY regulations of IEEE 802.11. An STAmay include an AP STA and a non-AP STA unless the functionality of theSTA is not individually different from that of an AP. In this case,assuming that communication is performed between an STA and an AP, theSTA may be interpreted as being a non-AP STA. In the example of FIG. 1,the STA 1, the STA 4, the STA 5, and the STA 6 correspond to non-APSTAs, and the STA 2 and the STA 3 correspond to AP STAs.

A non-AP STA corresponds to an apparatus directly handled by a user,such as a laptop computer or a mobile phone. In the followingdescription, a non-AP STA may also be called a wireless device, aterminal, user equipment (UE), a mobile station (MS), a mobile terminal,a wireless terminal, a wireless transmit/receive unit (WTRU), a networkinterface device, a machine-type communication (MTC) device, amachine-to-machine (M2M) device or the like.

Furthermore, an AP is a concept corresponding to a base station (BS), anode-B, an evolved Node-B (eNB), a base transceiver system (BTS), afemto BS or the like in other wireless communication fields.

Hereinafter, in this specification, downlink (DL) means communicationfrom an AP to a non-AP STA. Uplink (UL) means communication from anon-AP STA to an AP. In DL, a transmitter may be part of an AP, and areceiver may be part of a non-AP STA. In UL, a transmitter may be partof a non-AP STA, and a receiver may be part of an AP.

FIG. 2 is a diagram illustrating the configuration of layer architectureof an IEEE 802.11 system to which an embodiment of the present inventionmay be applied.

Referring to FIG. 2, the layer architecture of the IEEE 802.11 systemmay include an MAC sublayer and a PHY sublayer.

The PHY sublayer may be divided into a physical layer convergenceprocedure (PLCP) entity and a physical medium dependent (PMD) entity. Inthis case, the PLCP entity functions to connect the MAC sublayer and adata frame, and the PMD entity functions to wirelessly transmit andreceive data to and from two or more STAs.

The MAC sublayer and the PHY sublayer may include respective managemententities, which may be referred to as an MAC sublayer management entity(MLME) and a PHY sublayer management entity (PLME), respectively. Themanagement entities provide a layer management service interface throughthe operation of a layer management function. The MLME is connected tothe PLME and may perform the management operation of the MAC sublayer.Likewise, the PLME is also connected to the MLME and may perform themanagement operation of the PHY sublayer.

In order to provide a precise MAC operation, a station management entity(SME) may be present in each STA. The SME is a management entityindependent of each layer, and collects layer-based state informationfrom the MLME and the PLME or sets the values of layer-specificparameters. The SME may perform such a function instead of common systemmanagement entities and may implement a standard management protocol.

The MLME, the PLME, and the SME may interact with each other usingvarious methods based on primitives. More specifically, anXX-GET.request primitive is used to request the value of a managementinformation base (MIB) attribute. An XX-GET.confirm primitive returnsthe value of a corresponding MIB attribute if the state is “SUCCESS”,and indicates an error in the state field and returns the value in othercases. An XX-SET.request primitive is used to make a request so that adesignated MIB attribute is set as a given value. If an MIB attributemeans a specific operation, such a request requests the execution of thespecific operation. Furthermore, an XX-SET.confirm primitive means thata designated MIB attribute has been set as a requested value if thestate is “SUCCESS.” In other cases, the XX-SET.confirm primitiveindicates that the state field is an error situation. If an MIBattribute means a specific operation, the primitive may confirm that acorresponding operation has been performed.

An operation in each sublayer is described in brief as follows.

The MAC sublayer generates one or more MAC protocol data units (MPDUs)by attaching an MAC header and a frame check sequence (FCS) to a MACservice data unit (MSDU) received from a higher layer (e.g., an LLClayer) or the fragment of the MSDU. The generated MPDU is delivered tothe PHY sublayer.

If an aggregated MSDU (A-MSDU) scheme is used, a plurality of MSDUs maybe aggregated into a single aggregated MSDU (A-MSDU). The MSDUaggregation operation may be performed in an MAC higher layer. TheA-MSDU is delivered to the PHY sublayer as a single MPDU (if it is notfragmented).

The PHY sublayer generates a physical protocol data unit (PPDU) byattaching an additional field, including information for a PHYtransceiver, to a physical service data unit (PSDU) received from theMAC sublayer. The PPDU is transmitted through a wireless medium.

The PSDU has been received by the PHY sublayer from the MAC sublayer,and the MPDU has been transmitted from the MAC sublayer to the PHYsublayer. Accordingly, the PSDU is substantially the same as the MPDU.

If an aggregated MPDU (A-MPDU) scheme is used, a plurality of MPDUs (inthis case, each MPDU may carry an A-MSDU) may be aggregated in a singleA-MPDU. The MPDU aggregation operation may be performed in an MAC lowerlayer. The A-MPDU may include an aggregation of various types of MPDUs(e.g., QoS data, acknowledge (ACK), and a block ACK (BlockAck)). The PHYsublayer receives an A-MPDU, that is, a single PSDU, from the MACsublayer. That is, the PSDU includes a plurality of MPDUs. Accordingly,the A-MPDU is transmitted through a wireless medium within a singlePPDU.

Physical Protocol Data Unit (PPDU) Format

A PPDU means a data block generated in the physical layer. A PPDU formatis described below based on an IEEE 802.11 a WLAN system to which anembodiment of the present invention may be applied.

FIG. 3 illustrating a non-HT format PPDU and an HT format PPDU in awireless communication system to which an embodiment of the presentinvention may be applied.

FIG. 3(a) illustrates a non-HT format PPDU for supporting IEEE 802.11a/gsystems. The non-HT PPDU may also be called a legacy PPDU.

Referring to FIG. 3(a), the non-HT format PPDU includes a legacy formatpreamble, including a legacy (or non-HT) short training field (L-STF), alegacy (or non-HT) long training field (L-LTF), and a legacy (or non-HT)signal (L-SIG) field, and a data field.

The L-STF may include a short training orthogonal frequency divisionmultiplexing symbol (OFDM). The L-STF may be used for frame timingacquisition, automatic gain control (AGC), diversity detection, andcoarse frequency/time synchronization.

The L-LTF may include a long training OFDM symbol. The L-LTF may be usedfor fine frequency/time synchronization and channel estimation.

The L-SIG field may be used to send control information for thedemodulation and decoding of the data field.

The L-SIG field may include a rate field of four bits, a reserved fieldof 1 bit, a length field of 12 bits, a parity bit of 1 bit, and a signaltail field of 6 bits.

The rate field includes transfer rate information, and the length fieldindicates the number of octets of a PSDU.

FIG. 3(b) illustrates an HT mixed format PPDU for supporting both anIEEE 802.11n system and IEEE 802.11a/g system.

Referring to FIG. 3(b), the HT mixed format PPDU includes a legacyformat preamble including an L-STF, an L-LTF, and an L-SIG field, an HTformat preamble including an HT-signal (HT-SIG) field, a HT shorttraining field (HT-STF), and a HT long training field (HT-LTF), and adata field.

The L-STF, the L-LTF, and the L-SIG field mean legacy fields forbackward compatibility and are the same as those of the non-HT formatfrom the L-STF to the L-SIG field. An L-STA may interpret a data fieldthrough an L-LTF, an L-LTF, and an L-SIG field although it receives anHT mixed PPDU. In this case, the L-LTF may further include informationfor channel estimation to be performed by an HT-STA in order to receivethe HT mixed PPDU and to demodulate the L-SIG field and the HT-SIGfield.

An HT-STA may be aware of an HT mixed format PPDU using the HT-SIG fieldsubsequent to the legacy fields, and may decode the data field based onthe HT mixed format PPDU.

The HT-LTF may be used for channel estimation for the demodulation ofthe data field. IEEE 802.11n supports single user multi-input andmulti-output (SU-MIMO) and thus may include a plurality of HT-LTFs forchannel estimation with respect to each of data fields transmitted in aplurality of spatial streams.

The HT-LTF may include a data HT-LTF used for channel estimation for aspatial stream and an extension HT-LTF additionally used for fullchannel sounding. Accordingly, a plurality of HT-LTFs may be the same asor greater than number of transmitted spatial streams.

In the HT mixed format PPDU, the L-STF, the L-LTF, and the L-SIG fieldsare first transmitted so that an L-STA can receive the L-STF, the L-LTF,and the L-SIG fields and obtain data. Thereafter, the HT-SIG field istransmitted for the demodulation and decoding of data transmitted for anHT-STA.

An L-STF, an L-LTF, L-SIG, and HT-SIG fields are transmitted withoutperforming beamforming up to an HT-SIG field so that an L-STA and anHT-STA can receive a corresponding PPDU and obtain data. In an HT-STF,an HT-LTF, and a data field that are subsequently transmitted, radiosignals are transmitted through precoding. In this case, an HT-STF istransmitted so that an STA receiving a corresponding PPDU by performingprecoding may take into considerate a portion whose power is varied byprecoding, and a plurality of HT-LTFs and a data field are subsequentlytransmitted.

Table 1 below illustrates the HT-SIG field.

TABLE 1 FIELD BIT DESCRIPTION MCS 7 Indicate a modulation and codingscheme CBW 20/40 1 Set to “0” if a CBW is 20 MHz or 40 MHz orupper/lower Set to “1” if a CBW is 40 MHz HT length 16 Indicate thenumber of data octets within a PSDU Smoothing 1 Set to “1” if channelsmoothing is recommended Set to “0” if channel estimation is recommendedunsmoothingly for each carrier Not-sounding 1 Set to “0” if a PPDU is asounding PPDU Set to “1” if a PPDU is not a sounding PPDU Reserved 1 Setto “1” Aggregation 1 Set to “1” if a PPDU includes an A-MPDU Set to “0”if not Space-time 2 Indicate a difference between the number of space-block coding time streams (NSTS) and the number of spatial (STBC)streams (NSS) indicated by an MCS Set to “00” if an STBC is not used FECcoding 1 Set to “1” if low-density parity check (LDPC) is used Set to“0” if binary convolutional code (BCC) is used Short GI 1 Set to “1” ifa short guard interval (GI) is used after HT training Set to “0” if notNumber of 2 Indicate the number of extension spatial streams extension(NESSs) spatial Set to “0” if there is no NESS streams Set to “1” if thenumber of NESSs is 1 Set to “2” if the number of NESSs is 2 Set to “3”if the number of NESSs is 3 CRC 8 Include CRS for detecting an error ofa PPDU on the receiver side Tail bits 6 Used to terminate the trellis ofa convolutional decoder Set to “0”

FIG. 3(c) illustrates an HT-green field format PPDU (HT-GF format PPDU)for supporting only an IEEE 802.11n system.

Referring to FIG. 3(c), the HT-GF format PPDU includes an HT-GF-STF, anHT-LTF1, an HT-SIG field, a plurality of HT-LTF2s, and a data field.

The HT-GF-STF is used for frame timing acquisition and AGC.

The HT-LTF1 is used for channel estimation.

The HT-SIG field is used for the demodulation and decoding of the datafield.

The HT-LTF2 is used for channel estimation for the demodulation of thedata field. Likewise, an HT-STA uses SU-MIMO. Accordingly, a pluralityof the HT-LTF2s may be configured because channel estimation isnecessary for each of data fields transmitted in a plurality of spatialstreams.

The plurality of HT-LTF2s may include a plurality of data HT-LTFs and aplurality of extension HT-LTFs like the HT-LTF of the HT mixed PPDU.

In FIGS. 3(a) to 3(c), the data field is a payload and may include aservice field, a scrambled PSDU (PSDU) field, tail bits, and paddingbits. All of the bits of the data field are scrambled.

FIG. 3(d) illustrates a service field included in the data field. Theservice field has 16 bits. The 16 bits are assigned No. 0 to No. 15 andare sequentially transmitted from the No. 0 bit. The No. 0 bit to theNo. 6 bit are set to 0 and are used to synchronize a descrambler withina reception stage.

An IEEE 802.11ac WLAN system supports the transmission of a DLmulti-user multiple input multiple output (MU-MIMO) method in which aplurality of STAs accesses a channel at the same time in order toefficiently use a radio channel. In accordance with the MU-MIMOtransmission method, an AP may simultaneously transmit a packet to oneor more STAs that have been subjected to MIMO pairing.

Downlink multi-user transmission (DL MU transmission) means a technologyin which an AP transmits a PPDU to a plurality of non-AP STAs throughthe same time resources using one or more antennas.

Hereinafter, an MU PPDU means a PPDU which delivers one or more PSDUsfor one or more STAs using the MU-MIMO technology or the OFDMAtechnology. Furthermore, an SU PPDU means a PPDU having a format inwhich only one PSDU can be delivered or which does not have a PSDU.

For MU-MIMO transmission, the size of control information transmitted toan STA may be relatively larger than size of 802.11n controlinformation. Control information additionally required to supportMU-MIMO may include information indicating the number of spatial streamsreceived by each STA and information related to the modulation andcoding of data transmitted to each STA may correspond to the controlinformation, for example.

Accordingly, when MU-MIMO transmission is performed to provide aplurality of STAs with a data service at the same time, the size oftransmitted control information may be increased according to the numberof STAs which receive the control information.

In order to efficiently transmit the control information whose size isincreased as described above, a plurality of pieces of controlinformation required for MU-MIMO transmission may be divided into twotypes of control information: common control information that isrequired for all of STAs in common and dedicated control informationindividually required for a specific STA, and may be transmitted.

FIG. 4 illustrates a VHT format PPDU in a wireless communication systemto which an embodiment of the present invention may be applied.

FIG. 4(a) illustrates a VHT format PPDU for supporting an IEEE 802.11acsystem.

Referring to FIG. 4(a), the VHT format PPDU includes a legacy formatpreamble including an L-STF, an L-LTF, and an L-SIG field, a VHT formatpreamble including a VHT-signal-A (VHT-SIG-A) field, a VHT shorttraining field (VHT-STF), a VHT long training field (VHT-LTF), and aVHT-signal-B (VHT-SIG-B) field, and a data field.

The L-STF, the L-LTF, and the L-SIG field mean legacy fields forbackward compatibility and have the same formats as those of the non-HTformat. In this case, the L-LTF may further include information forchannel estimation which will be performed in order to demodulate theL-SIG field and the VHT-SIG-A field.

The L-STF, the L-LTF, the L-SIG field, and the VHT-SIG-A field may berepeated in a 20 MHz channel unit and transmitted. For example, when aPPDU is transmitted through four 20 MHz channels (i.e., an 80 MHzbandwidth), the L-STF, the L-LTF, the L-SIG field, and the VHT-SIG-Afield may be repeated every 20 MHz channel and transmitted.

A VHT-STA may be aware of the VHT format PPDU using the VHT-SIG-A fieldsubsequent to the legacy fields, and may decode the data field based onthe VHT-SIG-A field.

In the VHT format PPDU, the L-STF, the L-LTF, and the L-SIG field arefirst transmitted so that even an L-STA can receive the VHT format PPDUand obtain data. Thereafter, the VHT-SIG-A field is transmitted for thedemodulation and decoding of data transmitted for a VHT-STA.

The VHT-SIG-A field is a field for the transmission of controlinformation that is common to a VHT STAs that are MIMO-paired with anAP, and includes control information for interpreting the received VHTformat PPDU.

The VHT-SIG-A field may include a VHT-SIG-A1 field and a VHT-SIG-A2field.

The VHT-SIG-A1 field may include information about a channel bandwidth(BW) used, information about whether space time block coding (STBC) isapplied or not, a group identifier (ID) for indicating a group ofgrouped STAs in MU-MIMO, information about the number of streams used(the number of space-time streams (NSTS)/part association identifier(AID), and transmit power save forbidden information. In this case, thegroup ID means an identifier assigned to a target transmitting STA groupin order to support MU-MIMO transmission, and may indicate whether thepresent MIMO transmission method is MU-MIMO or SU-MIMO.

Table 2 illustrates the VHT-SIG-A1 field.

TABLE 2 FIELD BIT DESCRIPTION BW 2 Set to “0” if a BW is 20 MHz Set to“1” if a BW is 40 MHz Set to “2” if a BW is 80 MHz Set to “3” if a BW is160 MHz or 80 + 80 MHz Reserved 1 STBC 1 In the case of a VHT SU PPDU:Set to “1” if STBC is used Set to “0” if not In the case of a VHT MUPPDU: Set to “0” group ID 6 Indicate a group ID “0” or “63” indicates aVHT SU PPDU, but indicates a VHT MU PPDU if not NSTS/Partial 12 In thecase of a VHT MU PPDU, divide into 4 AID user positions “p” each havingthree bits “0” if a space-time stream is 0 “1” if a space-time stream is1 “2” if a space-time stream is 2 “3” if a space-time stream is 3 “4” ifa space-time stream is 4 In the case of a VHT SU PPDU, Upper 3 bits areset as follows: “0” if a space-time stream is 1 “1” if a space-timestream is 2 “2” if a space-time stream is 3 “3” if a space-time streamis 4 “4” if a space-time stream is 5 “5” if a space-time stream is 6 “6”if a space-time stream is 7 “7” if a space-time stream is 8 Lower 9 bitsindicate a partial AID. TXOP_PS_ 1 Set to “0” if a VHT AP permits anon-AP VHT NOT_ STA to switch to power save mode during ALLOWEDtransmission opportunity (TXOP) Set to “1” if not In the case of a VHTPPDU transmitted by a non-AP VHT STA Set to “1” Reserved 1

The VHT-SIG-A2 field may include information about whether a short guardinterval (GI) is used or not, forward error correction (FEC)information, information about a modulation and coding scheme (MCS) fora single user, information about the type of channel coding for multipleusers, beamforming-related information, redundancy bits for cyclicredundancy checking (CRC), the tail bits of a convolutional decoder andso on.

Table 3 illustrates the VHT-SIG-A2 field.

TABLE 3 FIELD BIT DESCRIPTION Short GI 1 Set to “0” if a short GI is notused in a data field Set to “1” if a short GI is used in a data fieldShort GI 1 Set to “1” if a short GI is used and an extra disambiguationsymbol is required for the payload of a PPDU Set to “0” if an extrasymbol is not required SU/MU coding 1 In the case of a VHT SU PPDU: Setto “0” in the case of binary convolutional code (BCC) Set to “1” in thecase of low-density parity check (LDPC) In the case of a VHT MU PPDU:Indicate coding used if the NSTS field of a user whose user position is“0” is not “0” Set to “0” in the case of BCC Set to “1” in the case ofPDPC Set to “1” as a reserved field if the NSTS field of a user whoseuser position is “0” is “0” LDPC Extra 1 Set to “1” if an extra OFDMsymbol is required OFDM symbol due to an PDPC PPDU encoding procedure(in the case of a SU PPDU) or the PPDU encoding procedure of at leastone PDPC user (in the case of a VHT MU PPDU) Set to “0” if not SU VHT 4In the case of a VHT SU PPDU: MCS/MU Indicate a VHT-MCS index coding Inthe case of a VHT MU PPDU: Indicate coding for user positions “1” to “3”sequentially from upper bits Indicate coding used if the NSTS field ofeach user is not “1” Set to “0” in the case of BCC Set to “1” in thecase of LDPC Set to “1” as a reserved field if the NSTS field of eachuser is Beamformed 1 In the case of a VHT SU PPDU: Set to “1” if abeamforming steering matrix is applied to SU transmission Set to “0” ifnot In the case of a VHT MU PPDU: Set to “1” as a reserved fieldReserved 1 CRC 8 Include CRS for detecting an error of a PPDU on thereceiver side Tail 6 Used to terminate the trellis of a convolutionaldecoder Set to “0”

The VHT-STF is used to improve AGC estimation performance in MIMOtransmission.

The VHT-LTF is used for a VHT-STA to estimate an MIMO channel. Since aVHT WLAN system supports MU-MIMO, the VHT-LTF may be configured by thenumber of spatial streams through which a PPDU is transmitted.Additionally, if full channel sounding is supported, the number ofVHT-LTFs may be increased.

The VHT-SIG-B field includes dedicated control information which isnecessary for a plurality of MU-MIMO-paired VHT-STAs to receive a PPDUand to obtain data. Accordingly, only when common control informationincluded in the VHT-SIG-A field indicates that a received PPDU is forMU-MIMO transmission, a VHT-STA may be designed to decode the VHT-SIG-Bfield. In contrast, if common control information indicates that areceived PPDU is for a single VHT-STA (including SU-MIMO), an STA may bedesigned to not decode the VHT-SIG-B field.

The VHT-SIG-B field includes a VHT-SIG-B length field, a VHT-MCS field,a reserved field, and a tail field.

The VHT-SIG-B length field indicates the length of an A-MPDU (prior toend-of-frame (EOF) padding). The VHT-MCS field includes informationabout the modulation, encoding, and rate-matching of each VHT-STA.

The size of the VHT-SIG-B field may be different depending on the type(MU-MIMO or SU-MIMO) of MIMO transmission and a channel bandwidth usedfor PPDU transmission.

FIG. 4(b) illustrates a VHT-SIG-B field according to a PPDU transmissionbandwidth.

Referring to FIG. 4(b), in 40 MHz transmission, VHT-SIG-B bits arerepeated twice. In 80 MHz transmission, VHT-SIG-B bits are repeated fourtimes, and padding bits set to 0 are attached.

In 160 MHz transmission and 80+80 MHz transmission, first, VHT-SIG-Bbits are repeated four times as in the 80 MHz transmission, and paddingbits set to 0 are attached. Furthermore, a total of the 117 bits isrepeated again.

In a system supporting MU-MIMO, in order to transmit PPDUs having thesame size to STAs paired with an AP, information indicating the size ofthe bits of a data field forming the PPDU and/or information indicatingthe size of bit streams forming a specific field may be included in theVHT-SIG-A field.

In this case, an L-SIG field may be used to effectively use a PPDUformat. A length field and a rate field which are included in the L-SIGfield and transmitted so that PPDUs having the same size are transmittedto all of STAs may be used to provide required information. In thiscase, additional padding may be required in the physical layer becausean MAC protocol data unit (MPDU) and/or an aggregate MAC PDU (A-MPDU)are set based on the bytes (or octets) of the MAC layer.

In FIG. 4, the data field is a payload and may include a service field,a scrambled PSDU, tail bits, and padding bits.

An STA needs to determine the format of a received PPDU because severalformats of PPDUs are mixed and used as described above.

In this case, the meaning that a PPDU (or a PPDU format) is determinedmay be various. For example, the meaning that a PPDU is determined mayinclude determining whether a received PPDU is a PPDU capable of beingdecoded (or interpreted) by an STA. Furthermore, the meaning that a PPDUis determined may include determining whether a received PPDU is a PPDUcapable of being supported by an STA. Furthermore, the meaning that aPPDU is determined may include determining that information transmittedthrough a received PPDU is which information.

This is described in more detail below with reference to the followingdrawings.

FIG. 5 is a diagram illustrating constellations for classifying theformats of PPDUs in a wireless communication system to which anembodiment of the present invention may be applied.

FIG. 5(a) illustrates the constellation of an L-SIG field included in anon-HT format PPDU, FIG. 5(b) illustrates a phase rotation for HT mixedformat PPDU detection, and FIG. 5(c) illustrates a phase rotation forVHT format PPDU detection.

In order to classify a non-HT format PPDU, an HT-GF format PPDU, an HTmixed format PPDU, and a VHT format PPDU, an STA uses an L-SIG field andthe phase of the constellation of OFDM symbols transmitted after theL-SIG field. That is, the STA may determine a PPDU format based on theL-SIG field of the received PPDU and/or the phase of the constellationof OFDM symbols transmitted after the L-SIG field.

Referring to FIG. 5(a), binary phase shift keying (BPSK) is used as OFDMsymbols forming an L-SIG field.

First, in order to determine an HT-GF format PPDU, an STA determineswhether a detected SIG field is an L-SIG field when the first SIG fieldis detected in a received PPDU. That is, the STA attempts decoding basedon a constellation, such as the example of FIG. 5(a). When the decodingfails, the STA may determine a corresponding PPDU to be not an HT-GFformat PPDU.

Next, in order to determine a non-HT format PPDU, an HT mixed formatPPDU, and a VHT format PPDU, the phase of the constellation of OFDMsymbols transmitted after the L-SIG field may be used. That is, a methodfor modulating the OFDM symbols transmitted after the L-SIG field may bedifferent. An STA may determine a PPDU format based on a modulationmethod for a field after the L-SIG field of the received PPDU.

Referring to FIG. 5(b), in order to determine an HT mixed format PPDU,the phases of two OFDM symbols transmitted after the L-SIG field in theHT mixed format PPDU may be used.

More specifically, the phases of an OFDM symbol #1 and OFDM symbol #2corresponding to an HT-SIG field transmitted after the L-SIG field inthe HT mixed format PPDU are counterclockwise rotated 90 degrees. Thatis, quadrature binary phase shift keying (QBPSK) is used as a method formodulating the OFDM symbol #1 and the OFDM symbol #2. A QBPSKconstellation may be a constellation whose phase has beencounterclockwise rotated 90 degrees based on a BPSK constellation.

An STA attempts decoding for a first OFDM symbol and second OFDM symbolcorresponding to an HT-SIG-A field transmitted after the L-SIG field ofthe received PPDU based on a constellation, such as the example of FIG.5(b). If the decoding is successful, the STA determines that thecorresponding PPDU is an HT format PPDU.

Next, in order to determine a non-HT format PPDU and a VHT format PPDU,the phase of the constellation of OFDM symbols transmitted after theL-SIG field may be used.

Referring to FIG. 5(c), in order to determine a VHT format PPDU, thephases of two OFDM symbols transmitted after the L-SIG field in the VHTformat PPDU may be used.

More specifically, the phase of an OFDM symbol #1 corresponding to aVHT-SIG-A field after the L-SIG field in the VHT format PPDU is notrotated, but the phase of an OFDM symbol #2 is counterclockwise rotated90 degrees. That is, BPSK is used as a modulation method for the OFDMsymbol #1, and QBPSK is used as a modulation method for the OFDM symbol#2.

An STA attempts decoding for the first OFDM symbol and second OFDMsymbol corresponding to the VHT-SIG field transmitted after the L-SIGfield of the received PPDU based on a constellation, such as the exampleof FIG. 5(c). If the decoding is successful, the STA may determine thatthe corresponding PPDU is a VHT format PPDU.

In contrast, if the decoding fails, the STA may determine thecorresponding PPDU is a non-HT format PPDU.

MAC Frame Format

FIG. 6 illustrates the format of an MAC frame for an IEEE 802.11 systemto which an embodiment of the present invention may be applied.

Referring to FIG. 6, the MAC frame (i.e., an MPDU) includes an MACheader, a frame body, and a frame check sequence (FCS).

The MAC Header is defined as an area, including a Frame Control field, aDuration/ID field, an Address 1 field, an Address 2 field, an Address 3field, a Sequence Control field, an Address 4 field, a QoS Controlfield, and an HT Control field.

The Frame Control field includes information about the characteristicsof a corresponding MAC frame. The Frame Control field is described indetail later.

The Duration/ID field may be implemented to have a different valuedepending on the type and subtype of a corresponding MAC frame.

If the type and subtype of a corresponding MAC frame is a PS-poll framefor a power save (PS) operation, the Duration/ID field may be configuredto include the association identifier (AID) of an STA that hastransmitted the frame. In the remaining cases, the Duration/ID field maybe configured to have a specific duration value depending on the typeand subtype of a corresponding MAC frame. Furthermore, if a frame is anMPDU included in an aggregate-MPDU (A-MPDU) format, the Duration/IDfield included in an MAC header may be configured to have the samevalue.

The Address 1 field to the Address 4 field are used to indicate a BSSID,a source address (SA), a destination address (DA), a transmittingaddress (TA) indicating the address of a transmitting STA, and areceiving address (RA) indicating the address of a receiving STA.

An Address field implemented as a TA field may be set as a bandwidthsignaling TA value. In this case, the TA field may indicate that acorresponding MAC frame includes additional information in a scramblingsequence. The bandwidth signaling TA may be represented as the MACaddress of an STA that sends a corresponding MAC frame, butindividual/group bits included in the MAC address may be set as aspecific value (e.g., “1”).

The Sequence Control field includes a sequence number and a fragmentnumber. The sequence number may indicate a sequence number assigned to acorresponding MAC frame. The fragment number may indicate the number ofeach fragment of a corresponding MAC frame.

The QoS Control field includes information related to QoS. The QoSControl field may be included if it indicates a QoS Data frame in asubtype subfield.

The HT Control field includes control information related to an HTand/or VHT transmission/reception scheme. The HT Control field isincluded in a control wrapper frame. Furthermore, the HT Control fieldis present in a QoS Data frame having an order subfield value of 1 and amanagement frame.

The frame body is defined as an MAC payload. Data to be transmitted in ahigher layer is placed in the frame body. The frame body has a varyingsize. For example, a maximum size of an MPDU may be 11454 octets, and amaximum size of a PPDU may be 5.484 ms.

The FCS is defined as an MAC footer and used for the error search of anMAC frame.

The first three fields (i.e., the Frame Control field, the Duration/IDfield, and Address 1 field) and the Last field (i.e., the FCS field)form a minimum frame format and are present in all of frames. Theremaining fields may be present only in a specific frame type.

FIG. 7 is a diagram illustrating a Frame Control field within the MACframe in a wireless communication system to which an embodiment of thepresent invention may be applied.

Referring to FIG. 7, the Frame Control field includes a Protocol Versionsubfield, a Type subfield, a Subtype subfield, a To DS subfield, a FromDS subfield, a More Fragments subfield, a Retry subfield, a PowerManagement subfield, a More Data subfield, a Protected Frame subfield,and an Order subfield.

The Protocol Version subfield may indicate the version of a WLANprotocol applied to a corresponding MAC frame.

The Type subfield and the Subtype subfield may be set to indicateinformation that identifies the function of a corresponding MAC frame.

The type of MAC frame may include the three types of management frames,control frames, and data frames.

Furthermore, each of the frame types may be divided into subtypes.

For example, the control frames may include request to send (RTS) frame,a clear-to-send (CTS) frame, an acknowledgment (ACK) frame, a PS-pollframe, a contention free (CF)-end frame, a CF-End+CF-ACK frame, a blockACK request (BAR) frame, a block ACK (BA) frame, a control wrapper(Control+HTcontrol)) frame, a VHT null data packet announcement (NDPA),and a beamforming report poll frame.

The management frames may include a beacon frame, an announcementtraffic indication message (ATIM) frame, a disassociation frame, anassociation request/response frame, a reassociation request/responseframe, a probe request/response frame, an authentication frame, adeauthentication frame, an action frame, an action no ACK frame, and atiming advertisement frame.

The To DS subfield and the From DS subfield may include information thatis necessary to analyze an Address 1 field to an Address 4 fieldincluded in a corresponding MAC frame header. In the case of the controlframe, both the To DS subfield and the From DS subfield are set to “0.”In the case of the management frame, the To DS subfield and the From DSsubfield may be sequentially set to “1” and “0” if a corresponding frameis a QoS management frame (QMF) and may be sequentially set to “0” and“0” if a corresponding frame is not a QMF.

The More Fragments subfield may indicate whether a fragment to betransmitted after a corresponding MAC frame is present or not. The MoreFragments subfield may be set to “1” if another fragment of a currentMSDU or MMPDU is present and may be set to “0” if another fragment of acurrent MSDU or MMPDU is not present.

The Retry subfield may indicate whether the transmission of acorresponding MAC frame is based on the retransmission of a previous MACframe. The Retry subfield may be set to “1” if the transmission of acorresponding MAC frame is based on the retransmission of a previous MACframe and may be set to “0” if the transmission of a corresponding MACframe is not based on the retransmission of a previous MAC frame.

The Power Management subfield may indicate power management mode of anSTA. The Power Management subfield may indicate that an STA switches topower saving mode if the Power Management subfield value is “1.”

The More Data subfield may indicate whether an MAC frame to beadditionally transmitted is present or not. The More Data subfield maybe set to “1” if an MAC frame to be additionally transmitted is presentand may be set to “0” if an MAC frame to be additionally transmitted isnot present.

The Protected Frame subfield may indicate whether a Frame Body field hasbeen encrypted. The Protected Frame subfield may be set to “1” if theFrame Body field includes information processed by a cryptographicencapsulation algorithm and may be set to “0” if the Frame Body fielddoes not include information processed by a cryptographic encapsulationalgorithm.

The pieces of information included in each of the aforementioned fieldsmay comply with the definition of the IEEE 802.11 system. Furthermore,the aforementioned fields correspond to an example of fields which maybe included in an MAC frame, but the present invention is not limitedthereto. That is, each of the aforementioned fields may be replaced withanother field or an additional field may be further included and all ofthe fields may not be essentially included.

FIG. 8 illustrates a VHT format of an HT Control field in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

Referring to FIG. 8, the HT Control field may include a VHT subfield, anHT control middle subfield, an AC constraint subfield, and a reversedirection grant (RDG)/more PPDU subfield.

The VHT subfield indicates whether the HT Control field has the formatof an HT Control field for VHT (VHT=1) or has the format of an HTControl field for HT (VHT=0). In FIG. 8, it is assumed that the HTControl field is an HT Control field for VHT (i.e., VHT=1). The HTControl field for VHT may be called a VHT Control field.

The HT control middle subfield may be implemented to a different formatdepending on the indication of a VHT subfield. The HT control middlesubfield is described in detail later.

The AC constraint subfield indicates whether the mapped access category(AC) of a reverse direction (RD) data frame is constrained to a singleAC.

The RDG/more PPDU subfield may be differently interpreted depending onwhether a corresponding field is transmitted by an RD initiator or an RDresponder.

Assuming that a corresponding field is transmitted by an RD initiator,the RDG/more PPDU subfield is set as “1” if an RDG is present, and theRDG/more PPDU subfield is set as “0” if an RDG is not present. Assumingthat a corresponding field is transmitted by an RD responder, theRDG/more PPDU subfield is set as “1” if a PPDU including thecorresponding subfield is the last frame transmitted by the RDresponder, and the RDG/more PPDU subfield is set as “0” if another PPDUis transmitted.

As described above, the HT control middle subfield may be implemented toa different format depending on the indication of a VHT subfield.

The HT control middle subfield of an HT Control field for VHT mayinclude a reserved bit subfield, a modulation and coding scheme (MCS)feedback request (MRQ) subfield, an MRQ sequence identifier(MSI)/space-time block coding (STBC) subfield, an MCS feedback sequenceidentifier (MFSI)/least significant bit (LSB) of group ID (GID-L)subfield, an MCS feedback (MFB) subfield, a most significant Bit (MSB)of group ID (GID-H) subfield, a coding type subfield, a feedbacktransmission type (FB Tx type) subfield, and an unsolicited MFBsubfield.

Table 4 illustrates a description of each subfield included in the HTcontrol middle subfield of the VHT format.

TABLE 4 SUBFIELD MEANING DEFINITION MRQ MCS request Set to “1” if MCSfeedback (solicited MFB) is not requested Set to “0” if not MSI MRQsequence An MSI subfield includes a sequence identifier number within arange of 0 to 6 to identify a specific request if an unsolicited MFBsubfield is set to “0” and an MRQ subfield is set to “1.” Include acompressed MSI subfield (2 bits) and an STBC indication subfield (1 bit)if an unsolicited MFB subfield is “1.” MFSI/GID-L MFB sequence AnMFSI/GID-L subfield includes the identifier/LSB received value of an MSIincluded of group ID within a frame related to MFB information if anunsolicited MFB subfield is set to “0.” An MFSI/GID-L subfield includesthe lowest three bits of a group ID of a PPDU estimated by an MFB if anMFB is estimated from an MU PPDU. MFB VHT N_STS, An MFB subfieldincludes recommended MCS, BW, SNR MFB. VHT-MCS = 15, NUM_STS = 7feedback indicates that feedback is not present. GID-H MSB of group AGID-H subfield includes the most ID significant bit 3 bits of a group IDof a PPDU whose solicited MFB has been estimated if an unsolicited MFBfield is set to “1” and MFB has been estimated from a VHT MU PPDU. Allof GID-H subfields are set to “1” if MFB is estimated from an SU PPDU.Coding Type Coding type or If an unsolicited MFB subfield is set to MFBresponse “1”, a coding type subfield includes the coding type (binaryconvolutional code (BCC) includes 0 and low-density parity check (LDPC)includes 1) of a frame whose solicited MFB has been estimated FB Tx TypeTransmission An FB Tx Type subfield is set to “0” if type of MFB anunsolicited MFB subfield is set to “1” response and MFB has beenestimated from an unbeamformed VHT PPDU. An FB Tx Type subfield is setto “1” if an unsolicited MFB subfield is set to “1” and MFB has beenestimated from a beamformed VHT PPDU. Unsolicited Unsolicited Set to “1”if MFB is a response to MRQ MFB MCS feedback Set to “0” if MFB is not aresponse to indicator MRQ

Furthermore, the MFB subfield may include the number of VHT space timestreams (NUM_STS) subfield, a VHT-MCS subfield, a bandwidth (BW)subfield, and a signal to noise ratio (SNR) subfield.

The NUM_STS subfield indicates the number of recommended spatialstreams. The VHT-MCS subfield indicates a recommended MCS. The BWsubfield indicates bandwidth information related to a recommended MCS.The SNR subfield indicates an average SNR value of data subcarriers andspatial streams.

The information included in each of the aforementioned fields may complywith the definition of an IEEE 802.11 system. Furthermore, each of theaforementioned fields corresponds to an example of fields which may beincluded in an MAC frame and is not limited thereto. That is, each ofthe aforementioned fields may be substituted with another field,additional fields may be further included, and all of the fields may notbe essentially included.

Medium Access Mechanism

In IEEE 802.11, communication is basically different from that of awired channel environment because it is performed in a shared wirelessmedium.

In a wired channel environment, communication is possible based oncarrier sense multiple access/collision detection (CSMA/CD). Forexample, when a signal is once transmitted by a transmission stage, itis transmitted up to a reception stage without experiencing great signalattenuation because there is no great change in a channel environment.In this case, when a collision between two or more signals is detected,detection is possible. The reason for this is that power detected by thereception stage becomes instantly higher than power transmitted by thetransmission stage. In a radio channel environment, however, sincevarious factors (e.g., signal attenuation is great depending on thedistance or instant deep fading may be generated) affect a channel, atransmission stage is unable to accurately perform carrier sensingregarding whether a signal has been correctly transmitted by a receptionstage or a collision has been generated.

Accordingly, in a WLAN system according to IEEE 802.11, a carrier sensemultiple access with collision avoidance (CSMA/CA) mechanism has beenintroduced as the basic access mechanism of MAC. The CAMA/CA mechanismis also called a distributed coordination function (DCF) of IEEE 802.11MAC, and basically adopts a “listen before talk” access mechanism. Inaccordance with such a type of access mechanism, an AP and/or an STAperform clear channel assessment (CCA) for sensing a radio channel or amedium for a specific time interval (e.g., a DCF inter-frame space(DIFS)) prior to transmission. If, as a result of the sensing, themedium is determined to be an idle state, the AP and/or the STA startsto transmit a frame through the corresponding medium. In contrast, if,as a result of the sensing, the medium is determined to be a busy state(or an occupied status), the AP and/or the STA do not start theirtransmission, may wait for a delay time (e.g., a random backoff period)for medium access in addition to the DIFS assuming that several STAsalready wait for in order to use the corresponding medium, and may thenattempt frame transmission.

Assuming that several STAs trying to transmit frames are present, theywill wait for different times because the STAs stochastically havedifferent backoff period values and will attempt frame transmission. Inthis case, a collision can be minimized by applying the random backoffperiod.

Furthermore, the IEEE 802.11 MAC protocol provides a hybrid coordinationfunction (HCF). The HCF is based on a DCF and a point coordinationfunction (PCF). The PCF is a polling-based synchronous access method,and refers to a method for periodically performing polling so that allof receiving APs and/or STAs can receive a data frame. Furthermore, theHCF has enhanced distributed channel access (EDCA) and HCF controlledchannel access (HCCA). In EDCA, a provider performs an access method forproviding a data frame to multiple users on a contention basis. In HCCA,a non-contention-based channel access method using a polling mechanismis used. Furthermore, the HCF includes a medium access mechanism forimproving the quality of service (QoS) of a WLAN, and may transmit QoSdata in both a contention period (CP) and a contention-free period(CFP).

FIG. 9 is a diagram illustrating a random backoff period and a frametransmission procedure in a wireless communication system to which anembodiment of the present invention may be applied.

When a specific medium switches from an occupied (or busy) state to anidle state, several STAs may attempt to transmit data (or frames). Inthis case, as a scheme for minimizing a collision, each of the STAs mayselect a random backoff count, may wait for a slot time corresponding tothe selected random backoff count, and may attempt transmission. Therandom backoff count has a pseudo-random integer value and may bedetermined as one of uniformly distributed values in 0 to a contentionwindow (CW) range. In this case, the CW is a CW parameter value. In theCW parameter, CW_min is given as an initial value. If transmission fails(e.g., if ACK for a transmitted frame is not received), the CW_min mayhave a twice value. If the CW parameter becomes CW_max, it may maintainthe CW_max value until data transmission is successful, and the datatransmission may be attempted. If the data transmission is successful,the CW parameter is reset to a CW_min value. The CW, CW_min, and CW_maxvalues may be set to 2^n−1 (n=0, 1, 2, . . . ,).

When a random backoff process starts, an STA counts down a backoff slotbased on a determined backoff count value and continues to monitor amedium during the countdown. When the medium is monitored as a busystate, the STA stops the countdown and waits. When the medium becomes anidle state, the STA resumes the countdown.

In the example of FIG. 9, when a packet to be transmitted in the MAC ofan STA 3 is reached, the STA 3 may check that a medium is an idle stateby a DIFS and may immediately transmit a frame.

The remaining STAs monitor that the medium is the busy state and wait.In the meantime, data to be transmitted by each of an STA 1, an STA 2,and an STA 5 may be generated. When the medium is monitored as an idlestate, each of the STAs waits for a DIFS and counts down a backoff slotbased on each selected random backoff count value.

The example of FIG. 9 shows that the STA 2 has selected the smallestbackoff count value and the STA 1 has selected the greatest backoffcount value. That is, FIG. 7 illustrates that the remaining backoff timeof the STA 5 is shorter than the remaining backoff time of the STA 1 ata point of time at which the STA 2 finishes a backoff count and startsframe transmission.

The STA 1 and the STA 5 stop countdown and wait while the STA 2 occupiesthe medium. When the occupation of the medium by the STA is finished andthe medium becomes an idle state again, each of the STA 1 and the STA 5waits for a DIFS and resumes the stopped backoff count. That is, each ofthe STA 1 and the STA 5 may start frame transmission after counting downthe remaining backoff slot corresponding to the remaining backoff time.The STA 5 starts frame transmission because the STA 5 has a shorterremaining backoff time than the STA 1.

While the STA 2 occupies the medium, data to be transmitted by an STA 4may be generated. In this case, from a standpoint of the STA 4, when themedium becomes an idle state, the STA 4 waits for a DIFS and counts downa backoff slot corresponding to its selected random backoff count value.

FIG. 9 shows an example in which the remaining backoff time of the STA 5coincides with the random backoff count value of the STA 4. In thiscase, a collision may be generated between the STA 4 and the STA 5. Whena collision is generated, both the STA 4 and the STA 5 do not receiveACK, so data transmission fails. In this case, each of the STA 4 and theSTA 5 doubles its CW value, select a random backoff count value, andcounts down a backoff slot.

The STA 1 waits while the medium is the busy state due to thetransmission of the STA 4 and the STA 5. When the medium becomes an idlestate, the STA 1 may wait for a DIFS and start frame transmission afterthe remaining backoff time elapses.

The CSMA/CA mechanism includes virtual carrier sensing in addition tophysical carrier sensing in which an AP and/or an STA directly sense amedium.

Virtual carrier sensing is for supplementing a problem which may begenerated in terms of medium access, such as a hidden node problem. Forthe virtual carrier sensing, the MAC of a WLAN system uses a networkallocation vector (NAV). The NAV is a value indicated by an AP and/or anSTA which now uses a medium or has the right to use the medium in orderto notify another AP and/or STA of the remaining time until the mediumbecomes an available state. Accordingly, a value set as the NAVcorresponds to the period in which a medium is reserved to be used by anAP and/or an STA that transmit corresponding frames. An STA thatreceives an NAV value is prohibited from accessing the medium during thecorresponding period. The NAV may be set based on the value of theDuration field of the MAC header of a frame, for example.

An AP and/or an STA may perform a procedure for exchanging a request tosend (RTS) frame and a clear to send (CTS) frame in order to providenotification that they will access a medium. The RTS frame and the CTSframe include information indicating a temporal section in which awireless medium required to transmit/receive an ACK frame has beenreserved to be accessed if substantial data frame transmission and anacknowledgement response (ACK) are supported. Another STA which hasreceived an RTS frame from an AP and/or an STA attempting to send aframe or which has received a CTS frame transmitted by an STA to which aframe will be transmitted may be configured to not access a mediumduring a temporal section indicated by information included in theRTS/CTS frame. This may be implemented by setting the NAV during a timeinterval.

Interframe Space (IFS)

A time interval between frames is defined as an interframe space (IFS).An STA may determine whether a channel is used during an IFS timeinterval through carrier sensing. In an 802.11 WLAN system, a pluralityof IFSs is defined in order to provide a priority level by which awireless medium is occupied.

FIG. 10 is a diagram illustrating an IFS relation in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

All of pieces of timing may be determined with reference to physicallayer interface primitives, that is, a PHY-TXEND.confirm primitive, aPHYTXSTART.confirm primitive, a PHY-RXSTART.indication primitive, and aPHY-RXEND.indication primitive.

An interframe space (IFS) depending on an IFS type is as follows.

a) A reduced interframe space (IFS) (RIFS)

b) A short interframe space (IFS) (SIFS)

c) A PCF interframe space (IFS) (PIFS)

d) A DCF interframe space (IFS) (DIFS)

e) An arbitration interframe space (IFS) (AIFS)

f) An extended interframe space (IFS) (EIFS)

Different IFSs are determined based on attributes specified by aphysical layer regardless of the bit rate of an STA. IFS timing isdefined as a time gap on a medium. IFS timing other than an AIFS isfixed for each physical layer.

The SIFS is used to transmits a PPDU including an ACK frame, a CTSframe, a Block ACK Request (BlockAckReq) frame, or a block ACK(BlockAck) frame, that is, an instant response to an A-MPDU, the secondor consecutive MPDU of a fragment burst, and a response from an STA withrespect to polling according to a PCF. The SIFS has the highestpriority. Furthermore, the SIFS may be used for the point coordinator offrames regardless of the type of frame during a non-contention period(CFP) time. The SIFS indicates the time prior to the start of the firstsymbol of the preamble of a next frame which is subsequent to the end ofthe last symbol of a previous frame or from signal extension (ifpresent).

SIFS timing is achieved when the transmission of consecutive frames isstarted in a Tx SIFS slot boundary.

The SIFS is the shortest in IFS between transmissions from differentSTAs. The SIFS may be used if an STA occupying a medium needs tomaintain the occupation of the medium during the period in which theframe exchange sequence is performed.

Other STAs required to wait so that a medium becomes an idle state for alonger gap can be prevented from attempting to use the medium becausethe smallest gap between transmissions within a frame exchange sequenceis used. Accordingly, priority may be assigned in completing a frameexchange sequence that is in progress.

The PIFS is used to obtain priority in accessing a medium.

The PIFS may be used in the following cases.

-   -   An STA operating under a PCF    -   An STA sending a channel switch announcement frame    -   An STA sending a traffic indication map (TIM) frame    -   A hybrid coordinator (HC) starting a CFP or transmission        opportunity (TXOP)    -   An HC or non-AP QoS STA, that is, a TXOP holder polled for        recovering from the absence of expected reception within a        controlled access phase (CAP)    -   An HT STA using dual CTS protection before sending CTS2    -   A TXOP holder for continuous transmission after a transmission        failure    -   A reverse direction (RD) initiator for continuous transmission        using error recovery    -   An HT AP during a PSMP sequence in which a power save multi-poll        (PSMP) recovery frame is transmitted    -   An HT AT performing CCA within a secondary channel before        sending a 40 MHz mask PPDU using EDCA channel access

In the illustrated examples, an STA using the PIFS starts transmissionafter a carrier sense (CS) mechanism for determining that a medium is anidle state in a Tx PIFS slot boundary other than the case where CCA isperformed in a secondary channel.

The DIFS may be used by an STA which operates to send a data frame(MPDU) and a MAC management protocol data unit management (MMPDU) frameunder the DCF. An STA using the DCF may transmit data in a TxDIFS slotboundary if a medium is determined to be an idle state through a carriersense (CS) mechanism after an accurately received frame and a backofftime expire. In this case, the accurately received frame means a frameindicating that the PHY-RXEND.indication primitive does not indicate anerror and an FCS indicates that the frame is not an error (i.e., errorfree).

An SIFS time (“aSIFSTime”) and a slot time (“aSlotTime”) may bedetermined for each physical layer. The SIFS time has a fixed value, butthe slot time may be dynamically changed depending on a change in thewireless delay time “aAirPropagationTime.”

The “aSIFSTime” is defined as in Equations 1 and 2 below.aSIFSTime(16μs)=aRxRFDelay(0.5)+aRxPLCPDelay(12.5)+aMACProcessingDelay(1 or<2)+aRxTxTurnaroundTime(<2)  [Equation 1]aRxTxTurnaroundTime=aTxPLCPDelay(1)+aRxTxSwitchTime(0.25)+aTxRampOnTime(0.25)+aTxRFDelay(0.5)  [Equation2]

The “aSlotTime” is defined as in Equation 3 below.aSlotTime=aCCATime(<4)+aRxTxTurnaroundTime(<2)+aAirPropagationTime(<1)+aMACProoessingDelay(<2)  [Equation3]

In Equation 3, a default physical layer parameter is based on“aMACProcessingDelay” having a value which is equal to or smaller than 1μs. A radio wave is spread 300 m/μs in the free space. For example, 3 μsmay be the upper limit of a BSS maximum one-way distance ˜450 m (a roundtrip is ˜900 m).

The PIFS and the SIFS are defined as in Equations 4 and 5, respectively.DIFS(16 μs)=aSIFSTime+aSlotTime  [Equation 4]DIFS(34 μs)=aSIFSTime+2*aSlotTime  [Equation 5]

In Equations 1 to 5, the numerical value within the parenthesisillustrates a common value, but the value may be different for each STAor for the position of each STA.

The aforementioned SIFS, PIFS, and DIFS are measured based on an MACslot boundary (e.g., a Tx SIFS, a Tx PIFS, and a TxDIFS) different froma medium.

The MAC slot boundaries of the SIFS, the PIFS, and the DIFS are definedas in Equations 6 to 8, respectively.TxSIFS=SIFS−aRxTxTurnaroundTime  [Equation 6]TxPIFS=TxSIFS+aSlotTime  [Equation 7]TxDIFS=TxSIFS+2*aSlotTlme  [Equation 8]

Downlink (DL) MU-MIMO Frame

FIG. 11 is a diagram illustrating a DL multi-user (MU) PPDU format in awireless communication system to which an embodiment of the presentinvention may be applied.

Referring to FIG. 11, the PPDU includes a preamble and a data field. Thedata field may include a service field, a scrambled PSDU field, tailbits, and padding bits.

An AP may aggregate MPDUs and transmit a data frame using an aggregatedMPDU (A-MPDU) format. In this case, a scrambled PSDU field may includethe A-MPDU.

The A-MPDU includes a sequence of one or more A-MPDU subframes.

In the case of a VHT PPDU, the length of each A-MPDU subframe is amultiple of 4 octets. Accordingly, an A-MPDU may include an end-of-frame(EOF) pad of 0 to 3 octets after the last A-MPDU subframe in order tomatch the A-MPDU up with the last octet of a PSDU.

The A-MPDU subframe includes an MPDU delimiter, and an MPDU may beoptionally included after the MPDU delimiter. Furthermore, a pad octetis attached to the MPDU in order to make the length of each A-MPDUsubframe in a multiple of 4 octets other than the last A-MPDU subframewithin one A-MPDU.

The MPDU delimiter includes a reserved field, an MPDU length field, acyclic redundancy check (CRC) field, and a delimiter signature field.

In the case of a VHT PPDU, the MPDU delimiter may further include anend-of-frame (EOF) field. If an MPDU length field is 0 and an A-MPDUsubframe or A-MPDU used for padding includes only one MPDU, in the caseof an A-MPDU subframe on which a corresponding MPDU is carried, the EOFfield is set to “1.” If not, the EOF field is set to “0.”

The MPDU length field includes information about the length of the MPDU.

If an MPDU is not present in a corresponding A-MPDU subframe, the PDUlength field is set to “0.” An A-MPDU subframe in which an MPDU lengthfield has a value of “0” is used to be padded to a corresponding A-MPDUin order to match the A-MPDU up with available octets within a VHT PPDU.

The CRC field includes CRC information for an error check. The delimitersignature field includes pattern information used to search for an MPDUdelimiter.

Furthermore, the MPDU includes an MAC header, a frame body, and a framecheck sequence (FCS).

FIG. 12 is a diagram illustrating a DL multi-user (MU) PPDU format in awireless communication system to which an embodiment of the presentinvention may be applied.

In FIG. 12, the number of STAs receiving a corresponding PPDU is assumedto be 3 and the number of spatial streams allocated to each STA isassumed to be 1, but the number of STAs paired with an AP and the numberof spatial streams allocated to each STA are not limited thereto.

Referring to FIG. 12, the MU PPDU includes L-TFs (i.e., an L-STF and anL-LTF), an L-SIG field, a VHT-SIG-A field, a VHT-TFs (i.e., a VHT-STFand a VHT-LTF), a VHT-SIG-B field, a service field, one or more PSDUs, apadding field, and a tail bit. The L-TFs, the L-SIG field, the VHT-SIG-Afield, the VHT-TFs, and the VHT-SIG-B field are the same as those ofFIG. 4, and a detailed description thereof is omitted.

Information for indicating PPDU duration may be included in the L-SIGfield. In the PPDU, PPDU duration indicated by the L-SIG field includesa symbol to which the VHT-SIG-A field has been allocated, a symbol towhich the VHT-TFs have been allocated, a field to which the VHT-SIG-Bfield has been allocated, bits forming the service field, bits forming aPSDU, bits forming the padding field, and bits forming the tail field.An STA receiving the PPDU may obtain information about the duration ofthe PPDU through information indicating the duration of the PPDUincluded in the L-SIG field.

As described above, group ID information and time and spatial streamnumber information for each user are transmitted through the VHT-SIG-A,and a coding method and MCS information are transmitted through theVHT-SIG-B. Accordingly, beamformees may check the VHT-SIG-A and theVHT-SIG-B and may be aware whether a frame is an MU MIMO frame to whichthe beamformee belongs. Accordingly, an STA which is not a member STA ofa corresponding group ID or which is a member of a corresponding groupID, but in which the number of streams allocated to the STA is “0” isconfigured to stop the reception of the physical layer to the end of thePPDU from the VHT-SIG-A field, thereby being capable of reducing powerconsumption.

In the group ID, an STA can be aware that a beamformee belongs to whichMU group and it is a user who belongs to the users of a group to whichthe STA belongs and who is placed at what place, that is, that a PPDU isreceived through which stream by previously receiving a group IDmanagement frame transmitted by a beamformer.

All of MPDUs transmitted within the VHT MU PPDU based on 802.11ac areincluded in the A-MPDU. In the data field of FIG. 12, each VHT A-MPDUmay be transmitted in a different stream.

In FIG. 12, the A-MPDUs may have different bit sizes because the size ofdata transmitted to each STA may be different.

In this case, null padding may be performed so that the time when thetransmission of a plurality of data frames transmitted by a beamformeris ended is the same as the time when the transmission of a maximuminterval transmission data frame is ended. The maximum intervaltransmission data frame may be a frame in which valid downlink data istransmitted by a beamformer for the longest time. The valid downlinkdata may be downlink data that has not been null padded. For example,the valid downlink data may be included in the A-MPDU and transmitted.Null padding may be performed on the remaining data frames other thanthe maximum interval transmission data frame of the plurality of dataframes.

For the null padding, a beamformer may fill one or more A-MPDUsubframes, temporally placed in the latter part of a plurality of A-MPDUsubframes within an A-MPDU frame, with only an MPDU delimiter fieldthrough encoding. An A-MPDU subframe having an MPDU length of 0 may becalled a null subframe.

As described above, in the null subframe, the EOF field of the MPDUdelimiter is set to “1.” Accordingly, when the EOF field set to 1 isdetected in the MAC layer of an STA on the receiving side, the receptionof the physical layer is stopped, thereby being capable of reducingpower consumption.

Block ACK Procedure

FIG. 13 is a diagram illustrating a DL MU-MIMO transmission process in awireless communication system to which an embodiment of the presentinvention may be applied.

In 802.11ac, MU-MIMO is defined in downlink from an AP to a client(i.e., a non-AP STA). In this case, a multi-user (MU) frame istransmitted to multiple recipients at the same time, but acknowledgement(ACK) needs to be individually transmitted in uplink.

All of MPDUs transmitted within a VHT MU PPDU based on 802.11ac areincluded in an A-MPDU. Accordingly, a response to the A-MPDU within theVHT MU PPDU other than an immediate response to the VHT MU PPDU istransmitted in response to a block ACK request (BAR) frame by the AP.

First, an AP sends a VHT MU PPDU (i.e., a preamble and data) to all ofrecipients (i.e., an STA 1, an STA 2, and an STA 3). The VHT MU PPDUincludes a VHT A-MPDU transmitted to each of the STAs.

The STA 1 that has received the VHT MU PPDU from the AP sends a blockacknowledgement (BA) frame to the AP after an SIFS. The BA frame isdescribed later in detail.

The AP that has received the BA from the STA 1 sends a blockacknowledgement request (BAR) frame to the STA 2 after an SIFS. The STA2 sends a BA frame to the AP after an SIFS. The AP that has received theBA frame from the STA 2 sends a BAR frame to the STA 3 after an SIFS.The STA 3 sends a BA frame to the AP after an SIFS.

When such a process is performed by all of the STAs, the AP sends a nextMU PPDU to all of the STAs.

Acknowledgement (ACK)/Block ACK Frame

In general, an ACK frame is used as a response to an MPDU, and a blockACK frame is used as a response to an A-MPDU.

FIG. 14 is a diagram illustrating an ACK frame in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

Referring to FIG. 14, the ACK frame includes a Frame Control field, aDuration field, an RA field, and an FCS.

The RA field is set as the value of the second address (Address 2) fieldof a data frame, a management frame, a block ACK request frame, a blockACK frame or a PS-Poll frame that has been received right before.

When an ACK frame is transmitted by a non-QoS STA, if a more fragmentssubfield within the Frame Control field of a data frame or a managementframe that has been received right before is “0”, a duration value isset to “0.”

In an ACK frame not transmitted by a non-QoS STA, a duration value isset as a value (ms) obtained by subtracting the time required to sendthe ACK frame and an SIFS interval from the Duration/ID field of a dataframe, a management frame, a block ACK request frame, a block ACK frameor a PS-Poll frame that has been received right before. If thecalculated duration value is not an integer value, it is rounded off tothe nearest whole number.

Hereinafter, a block ACK (request) frame is described.

FIG. 15 is a diagram illustrating a block ACK request frame in awireless communication system to which an embodiment of the presentinvention may be applied.

Referring to FIG. 15, the Block ACK Request (BAR) frame includes a FrameControl field, a Duration/ID field, a Receiving Address (RA) field, aTransmitting Address (TA) field, a BAR Control field, a BAR Informationfield, and a frame check sequence (FCS).

The RA field may be set as the address of an STA that receives the BARframe.

The TA field may be set as the address of an STA that sends the BARframe.

The BAR Control field includes a BAR ACK Policy subfield, a Multi-TIDsubfield, a Compressed Bitmap subfield, a Reserved subfield, and a TIDInformation (TID Info) subfield.

Table 5 illustrates the BAR Control field.

TABLE 5 SUBFIELD BIT DESCRIPTION BAR ACK 1 Set to “0” when a senderrequests an immediate policy ACK for data transmission. Set to “1” whena sender does not request an immediate ACK for data transmission.Multi-TID 1 Indicate the type of BAR frame depending on the Compressed 1value of a Multi-TID subfield and a Compressed bitmap Bitmap subfield.00: Basic BAR 01: Compressed BAR 10: Reserved value 11: Multi-TID BARReserved 9 TID Info 4 The meaning of a TID Info field is determined bythe type of BAR frame. Include TID that requests a BA frame in the caseof a Basic BAR frame or a Compressed BAR frame. Include the number ofTIDs in the case of a Multi-TID BAR frame

The BAR Information field includes different information depending onthe type of BAR frame. This is described with reference to FIG. 16.

FIG. 16 is a diagram illustrating the BAR Information field of a blockACK request frame in a wireless communication system to which anembodiment of the present invention may be applied.

FIG. 16(a) illustrates the BAR information field of a Basic BAR frameand a Compressed BAR frame, and FIG. 16(b) illustrates a BAR informationfield of a Multi-TID BAR frame.

Referring to FIG. 16(a), in the case of the Basic BAR frame and theCompressed BAR frame, a BAR Information field includes a Block ACKStarting Sequence Control subfield.

Furthermore, the Block ACK Starting Sequence Control subfield includes aFragment Number subfield and a Starting Sequence Number subfield.

The Fragment Number field is set to 0.

In the case of the Basic BAR frame, the Starting Sequence Numbersubfield includes the sequence number of the first MSDU in which acorresponding BAR frame is transmitted. In the case of the CompressedBAR frame, the Starting Sequence Control subfield includes the sequencenumber of the first MSDU or A-MSDU for transmitting a corresponding BARframe.

Referring to FIG. 16(b), in the case of the Multi-TID BAR frame, the BARInformation field is configured in such a manner that a Per TID Infosubfield and a Block ACK Starting Sequence Control subfield are repeatedfor one or more TIDs.

The Per TID Info subfield includes a Reserved subfield and a TID Valuesubfield. The TID Value subfield includes a TID value.

The Block ACK Starting Sequence Control subfield, as described above,includes a fragment number and a Starting Sequence Number subfield. TheFragment Number field is set to 0. The Starting Sequence Controlsubfield includes the sequence number of the first MSDU or A-MSDU fortransmitting a corresponding BAR frame.

FIG. 17 is a diagram illustrating a block ACK frame in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

Referring to FIG. 17, the Block ACK (BA) frame includes a Frame Controlfield, a Duration/ID field, a Reception Address (RA) field, aTransmitting Address (TA) field, a BA Control field, a BA Informationfield, and a frame check sequence (FCS).

The RA field may be set as the address of an STA that has requested ablock ACK.

The TA field may be set as the address of an STA that sends a BA frame.

The BA Control field includes a BA ACK Policy subfield, a Multi-TIDsubfield, a Compressed Bitmap subfield, a Reserved subfield, and a TIDInformation (TID Info) subfield.

Table 6 illustrates the BA Control field.

TABLE 6 SUBFIELD BIT DESCRIPTION BA ACK 1 Set to “0” when a senderrequests an immediate policy ACK for data transmission. Set to “1” whena sender does not request an immediate ACK for data transmission.Multi-TID 1 Indicate the type of BA frame depending on the Compressed 1values of a Multi-TID subfield and a Compressed bitmap Bitmap subfield.00: Basic BA 01: Compressed BA 10: Reserved value 11: Multi-TID BAReserved 9 TID_Info 4 The meaning of a TID Info field is determined bythe type of BA frame. Include TID that requests a BA frame in the caseof a Basic BA frame, a Compressed BA frame. Include the number of TIDsin the case of a Multi-TID BA frame

The BA Information field includes different information depending on thetype of BA frame. This is described below with reference to FIG. 18.

FIG. 18 is a diagram illustrating the BA Information field of the blockACK frame in a wireless communication system to which an embodiment ofthe present invention may be applied.

FIG. 18(a) illustrates the BA Information field of a Basic BA frame,FIG. 18(b) illustrates the BA Information field of a Compressed BAframe, and FIG. 18(c) illustrates the BA Information field of aMulti-TID BA frame.

Referring to FIG. 18(a), in the case of the Basic BA frame, the BAInformation field includes a Block ACK Starting Sequence Controlsubfield and a Block ACK Bitmap subfield.

The Block ACK Starting Sequence Control subfield includes a FragmentNumber field and a Starting Sequence Number subfield as described above.

The Fragment Number field is set to 0.

The Starting Sequence Number subfield includes the sequence number ofthe first MSDU for transmitting a corresponding BA frame, and is set asthe same value as the Basic BAR frame that has been received rightbefore.

The Block ACK Bitmap subfield has the length of 128 octets and is usedto indicate the reception state of a maximum of 64 MSDUs. In the BlockACK Bitmap subfield, a value “1” indicates that an MPDU corresponding toa corresponding bit location has been successfully received. A value “0”indicates that an MPDU corresponding to a corresponding bit location hasnot been successfully received.

Referring to FIG. 18(b), in the case of the Compressed BA frame, the BAInformation field includes a Block ACK Starting Sequence Controlsubfield and a Block ACK Bitmap subfield.

The Block ACK Starting Sequence Control subfield includes a FragmentNumber field and a Starting Sequence Number subfield as described above.

The Fragment Number field is set to 0.

The Starting Sequence Number subfield includes the sequence number ofthe first MSDU or A-MSDU for transmitting a corresponding BA frame, andis set as the same value as the Basic BAR frame that has been receivedright before.

The Block ACK Bitmap subfield has the length of 8 octets and is used toindicate the reception state a maximum of 64 MSDUs and A-MSDUs. In theBlock ACK Bitmap subfield, a value “1” indicates that a single MSDU orA-MSDU corresponding to a corresponding bit location has beensuccessfully received. A value “0” indicates that a single MSDU orA-MSDU corresponding to a corresponding bit location has not beensuccessfully received.

Referring to FIG. 18(c), in the case of the Multi-TID BA frame, the BAInformation field is configured in such a manner that a Per TID Infosubfield, a Block ACK Starting Sequence Control subfield, and a BlockACK Bitmap subfield are repeated for one or more TIDs and is configuredin order of an increasing TID.

The Per TID Info subfield includes a Reserved subfield and a TID Valuesubfield. The TID Value subfield includes a TID value.

The Block ACK Starting Sequence Control subfield includes a fragmentnumber and a Starting Sequence Number subfield as described above. TheFragment Number field is set to 0. The Starting Sequence Controlsubfield includes the sequence number of the first MSDU or A-MSDU fortransmitting a corresponding BA frame.

The Block ACK Bitmap subfield has a length of 8 octets. In the Block ACKBitmap subfield, a value “1” indicates that a single MSDU or A-MSDUcorresponding to a corresponding bit location has been successfullyreceived. A value “0” indicates that a single MSDU or A-MSDUcorresponding to a corresponding bit location has not been successfullyreceived.

Uplink Single User/Multi-User Transmitting Method

A new frame format and numerology for an 802.11ax system, that is, thenext-generation WLAN system, are actively discussed in the situation inwhich vendors of various fields have lots of interests in thenext-generation Wi-Fi and a demand for high throughput and quality ofexperience (QoE) performance improvement are increased after 802.11ac.

IEEE 802.11ax is one of WLAN systems recently and newly proposed as thenext-generation WLAN systems for supporting a higher data rate andprocessing a higher user load, and is also called a so-called highefficiency WLAN (HEW).

An IEEE 802.11ax WLAN system may operate in a 2.4 GHz frequency band anda 5 GHz frequency band like the existing WLAN systems. Furthermore, theIEEE 802.11ax WLAN system may also operate in a higher 60 GHz frequencyband.

In the IEEE 802.11ax system, an FFT size four times larger than that ofthe existing IEEE 802.11 OFDM systems (e.g., IEEE 802.11a, 802.11n, and802.11ac) may be used in each bandwidth for average throughputenhancement and outdoor robust transmission for inter-symbolinterference. This is described below with reference to relateddrawings.

In the following description of an HE format PPDU according to anembodiment of the present invention, the descriptions of theaforementioned non-HT format PPDU, HT mixed format PPDU, HT-green fieldformat PPDU and/or VHT format PPDU may be reflected into the descriptionof the HE format PPDU although they are not described otherwise.

FIG. 19 is a diagram illustrating a high efficiency (HE) format PPDUaccording to an embodiment of the present invention.

FIG. 19(a) illustrates a schematic configuration of the HE format PPDU,and FIGS. 19(b) to 19(d) illustrate more detailed configurations of theHE format PPDU.

Referring to FIG. 19(a), the HE format PPDU for an HEW may basicallyinclude a legacy part (L-part), an HE-part, and an HE-data field.

The L-part includes an L-STF, an L-LTF, and an L-SIG field as in a formmaintained in the existing WLAN system. The L-STF, the L-LTF, and theL-SIG field may be called a legacy preamble.

The HE-part is a part newly defined for the 802.11ax standard and mayinclude an HE-STF, an HE-SIG field, and an HE-LTF. In FIG. 19(a), thesequence of the HE-STF, the HE-SIG field, and the HE-LTF is illustrated,but the HE-STF, the HE-SIG field, and the HE-LTF may be configured in adifferent sequence. Furthermore, the HE-LTF may be omitted. Not only theHE-STF and the HE-LTF, but the HE-SIG field may be commonly called anHE-preamble.

The HE-SIG may include information (e.g., OFDMA, UL MU MIMO, andimproved MCS) for decoding the HE-data field.

The L-part and the HE-part may have different fast Fourier transform(FFT) sizes (i.e., different subcarrier spacing) and use differentcyclic prefixes (CPs).

In an 802.11ax system, an FFT size four times (4×) larger than that of alegacy WLAN system may be used. That is, the L-part may have a 1× symbolstructure, and the HE-part (more specifically, HE-preamble and HE-data)may have a 4× symbol structure. In this case, the FFT of a 1×, 2×, or 4×size means a relative size for a legacy WLAN system (e.g., IEEE 802.11a,802.11n, and 802.11ac).

For example, if the sizes of FFTs used in the L-part are 64, 128, 256,and 512 in 20 MHz, 40 MHz, 80 MHz, and 160 MHz, respectively, the sizesof FFTs used in the HE-part may be 256, 512, 1024, and 2048 in 20 MHz,40 MHz, 80 MHz, and 160 MHz, respectively.

If an FFT size is larger than that of a legacy WLAN system as describedabove, subcarrier frequency spacing is reduced. Accordingly, the numberof subcarriers per unit frequency is increased, but the length of anOFDM symbol is increased.

That is, if a larger FFT size is used, it means that subcarrier spacingis narrowed. Likewise, it means that an inverse discrete Fouriertransform (IDFT)/discrete Fourier transform (DFT) period is increased.In this case, the IDFT/DFT period may mean a symbol length other than aguard interval (GI) in an OFDM symbol.

Accordingly, if an FFT size four times larger than that of the L-part isused in the HE-part (more specifically, the HE-preamble and the HE-datafield), the subcarrier spacing of the HE-part becomes ¼ times thesubcarrier spacing of the L-part, and the IDFT/DFT period of the HE-partis four times the IDFT/DFT period of the L-part. For example, if thesubcarrier spacing of the L-part is 312.5 kHz (=20 MHz/64, 40 MHz/128,80 MHz/256 and/or 160 MHz/512), the subcarrier spacing of the HE-partmay be 78.125 kHz (=20 MHz/256, 40 MHz/512, 80 MHz/1024 and/or 160MHz/2048). Furthermore, if the IDFT/DFT period of the L-part is 3.2 μs(=1/312.5 kHz), the IDFT/DFT period of the HE-part may be 12.8 μs(=1/78.125 kHz).

In this case, since one of 0.8 μs, 1.6 μs, and 3.2 μs may be used as aGI, the OFDM symbol length (or symbol interval) of the HE-part includingthe GI may be 13.6 μs, 14.4 μs, or 16 μs depending on the GI.

Referring to FIG. 19(b), the HE-SIG field may be divided into anHE-SIG-A field and an HE-SIG-B field.

For example, the HE-part of the HE format PPDU may include an HE-SIG-Afield having a length of 12.8 μs, an HE-STF of 1 OFDM symbol, one ormore HE-LTFs, and an HE-SIG-B field of 1 OFDM symbol.

Furthermore, in the HE-part, an FFT size four times larger than that ofthe existing PPDU may be applied from the HE-STF other than the HE-SIG-Afield. That is, FFTs having 256, 512, 1024, and 2048 sizes may beapplied from the HE-STFs of the HE format PPDUs of 20 MHz, 40 MHz, 80MHz, and 160 MHz, respectively.

In this case, if the HE-SIG field is divided into the HE-SIG-A field andthe HE-SIG-B field as in FIG. 19(b), the positions of the HE-SIG-A fieldand the HE-SIG-B field may be different from those of FIG. 19(b). Forexample, the HE-SIG-B field may be transmitted after the HE-SIG-A field,and the HE-STF and the HE-LTF may be transmitted after the HE-SIG-Bfield. In this case, an FFT size four times larger than that of theexisting PPDU may be applied from the HE-STF.

Referring to FIG. 19(c), the HE-SIG field may not be divided into anHE-SIG-A field and an HE-SIG-B field.

For example, the HE-part of the HE format PPDU may include an HE-STF of1 OFDM symbol, an HE-SIG field of 1 OFDM symbol, and one or moreHE-LTFs.

In the manner similar to that described above, an FFT size four timeslarger than that of the existing PPDU may be applied to the HE-part.That is, FFT sizes of 256, 512, 1024, and 2048 may be applied from theHE-STF of the HE format PPDU of 20 MHz, 40 MHz, 80 MHz, and 160 MHz,respectively.

Referring to FIG. 19(d), the HE-SIG field is not divided into anHE-SIG-A field and an HE-SIG-B field, and the HE-LTF may be omitted.

For example, the HE-part of the HE format PPDU may include an HE-STF of1 OFDM symbol and an HE-SIG field of 1 OFDM symbol.

In the manner similar to that described above, an FFT size four timeslarger than that of the existing PPDU may be applied to the HE-part.That is, FFT sizes of 256, 512, 1024, and 2048 may be applied from theHE-STF of the HE format PPDU of 20 MHz, 40 MHz, 80 MHz, and 160 MHz,respectively.

The HE format PPDU for a WLAN system according to an embodiment of thepresent invention may be transmitted through at least one 20 MHzchannel. For example, the HE format PPDU may be transmitted in a 40 MHz,80 MHz or 160 MHz frequency band through a total of four 20 MHzchannels. This is described in more detail.

An HE format PPDU for a WLAN system to which an embodiment of thepresent invention may be applied may be transmitted through at least one20 MHz channel. For example, the HE format PPDU may be transmitted in a40 MHz, 80 MHz or 160 MHz frequency band through a total of four 20 MHzchannels. This is described in more detail below with reference to thefollowing drawing.

The following PPDU format is described based on FIG. 25(b), forconvenience of description, but the present invention is not limitedthereto.

FIG. 20 is a diagram illustrating an HE format PPDU according to anembodiment of the present invention.

FIG. 20 illustrates a PPDU format if an 80 MHz frequency band has beenallocated to one STA (or if an OFDMA resource unit has been allocated toa plurality of STAs within an 80 MHz frequency band) or if differentstreams each having an 80 MHz frequency band have been allocated to aplurality of STAs.

Referring to FIG. 20, an L-STF, an L-LTF, and an L-SIG field may betransmitted in an OFDM symbol generated based on 64 FFT points (or 64subcarriers) in each 20 MHz channel.

An HE-SIG A field may include common control information in commontransmitted to STAs that receive a PPDU. The HE-SIG A field may betransmitted in one to three OFDM symbols. The HE-SIG A field may beduplicated in a 20 MHz unit and includes the same information.Furthermore, the HE-SIG-A field provides notification of informationabout the full bandwidth of a system.

Table 7 illustrates information included in the HE-SIG A field.

TABLE 7 FIELD BIT DESCRIPTION Bandwidth 2 Indicates a bandwidth in whicha PPDU is transmitted. For example, 20 MHz, 40 MHz, 80 MHz or 160 MHzGroup ID 6 Indicates an STA or a group of STAs that will receive a PPDUStream 12 Indicate the position or number of spatial streams informationfor each STA or indicate the position or number of spatial streams for agroup of STAs UL 1 Indicate whether a PPDU is directed toward anindication AP (uplink) or an STA (downlink) MU 1 Indicate whether a PPDUis an SU-MIMO PPDU indication or an MU-MIMO PPDU GI indication 1Indicate whether a short GI or a long GI is used Allocation 12 Indicatea band or channel (a subchannel index or information subband index)allocated to each STA in a band in which a PPDU is transmittedTransmission 12 Indicate transmission power for each channel or powereach STA

Pieces of information included in each of the fields illustrated inTable 7 may comply with the definition of the IEEE 802.11 system.Furthermore, each of the aforementioned fields corresponds to an exampleof fields which may be included in a PPDU, but is not limited thereto.That is, each of the aforementioned fields may be replaced with anotherfield or an additional field may be further included and all of thefields may not be essentially included.

The HE-STF is used to improve performance of AGC estimation in MIMOtransmission.

The HE-SIG B field may include user-specific information which isrequired for each of STAs to receive its data (e.g., a PSDU). The HE-SIGB field may be transmitted in one or two OFDM symbols. For example, theHE-SIG B field may include information about a modulation and codingscheme (MCS) for a corresponding PSDU and the length of thecorresponding PSDU.

The L-STF, L-LTF, the L-SIG field, and the HE-SIG A field may berepeated in a 20 MHz channel unit and transmitted. For example, when aPPDU is transmitted through four 20 MHz channels (i.e., 80 MHz bands),the L-STF, the L-LTF, the L-SIG field, and the HE-SIG A field may berepeated every 20 MHz channel and transmitted.

If the size of FFT increases, a legacy STA supporting the existing IEEE802.11a/g/n/ac may not decode a corresponding HE PPDU. In order for alegacy STA and an HE STA to coexist, the L-STF, L-LTF, and the L-SIGfield are transmitted through 64 FFT in a 20 MHz channel so that alegacy STA can receive them. For example, the L-SIG field may occupy oneOFDM symbol, and one OFDM symbol time may be 4 μs, and a GI may be 0.8μs.

The size of FFT for each frequency unit may be further increased fromthe HE-STF (or the HE-SIG A field). For example, 256 FFT may be used ina 20 MHz channel, 512 FFT may be used in a 40 MHz channel, and 1024 FFTmay be used in an 80 MHz channel. If the size of FFT increases, thenumber of OFDM subcarriers per unit frequency increases because spacingbetween the OFDM subcarriers is reduced, but the OFDM symbol time isincreased. In order to improve efficiency of a system, the length of aGI after the HE-STF may be set to be the same as that of the HE-SIG Afield.

The HE-SIG A field may include information which is required for an HESTA to decode an HE PPDU. However, the HE-SIG A field may be transmittedthrough 64 FFT in a 20 MHz channel so that both a legacy STA and an HESTA can receive the HE-SIG A field. The reason for this is that the HESTA has to receive an existing HT/VHT format PPDU in addition to an HEformat PPDU and a legacy STA and the HE STA have to distinguish theHT/VHT format PPDU from the HE format PPDU.

FIG. 21 is a diagram illustrating a HE format PPDU according to anembodiment of the present invention.

In FIG. 21, it is assumed that 20 MHz channels are allocated todifferent STAs (e.g., an STA 1, an STA 2, an STA 3, and an STA 4).

Referring to FIG. 21, in this case, the size of FFT per unit frequencymay be further increased after an HE-STF (or the HE-SIG B field). Forexample, 256 FFT may be used in a 20 MHz channel, 512 FFT may be used ina 40 MHz channel, and 1024 FFT may be used in an 80 MHz channel from theHE-STF (or the HE-SIG B field).

Information transmitted in each of the fields included in the HE formatPPDU is the same as that of FIG. 26, and a description thereof isomitted.

The HE-SIG B field may include information specified for each of theSTAs, but may be encoded in a full band (i.e., indicated in the HE-SIG-Afield). That is, the HE-SIG B field includes information about all ofthe STAs and is received by all of the STAs.

The HE-SIG B field may provide notification of information about afrequency bandwidth allocated to each of the STAs and/or streaminformation in a corresponding frequency band. For example, in theHE-SIG-B field of FIG. 28, a 20 MHz frequency band may be allocated tothe STA 1, a next 20 MHz frequency band may be allocated to the STA 2, anext 20 MHz frequency band may be allocated to the STA 3, and a next 20MHz frequency band may be allocated to the STA 4. Furthermore, a 40 MHzfrequency band may be allocated to the STA 1 and the STA 2, and a next40 MHz frequency band may be allocated to the STA 3 and the STA 4. Inthis case, different streams may be allocated to the STA 1 and the STA2, and different streams may be allocated to the STA 3 and the STA 4.

Furthermore, an HE-SIG-C field may be defined and added to the exampleof FIG. 27. In this case, in the HE-SIG-B field, information about allof the STAs may be transmitted in a full band, and control informationspecific to each of the STAs may be transmitted in a 20 MHz unit throughthe HE-SIG-C field.

Furthermore, in the examples of FIGS. 20 and 21, the HE-SIG-B field isnot transmitted in a full band, but may be transmitted in a 20 MHz unitlike the HE-SIG-A field. This is described below with reference torelated drawings.

FIG. 22 is a diagram illustrating a HE format PPDU according to anembodiment of the present invention.

In FIG. 22, it is assumed that 20 MHz channels are allocated todifferent STAs (e.g., STA 1, STA 2, STA 3, and STA 4).

Referring to FIG. 22, the HE-SIG-B field is not transmitted over anentire band but transmitted in a unit of 20 MHz, as in the HE-SIG-Afield. However, in this case, the HE-SIG-B is encoded in a unit of 20MHz and transmitted, unlike the HE-SIG-A field, but the HE-SIG-B may notbe duplicated in a unit of 20 MHz and transmitted.

In this case, the FFT size per unit frequency may further increase froman HE-STF (or HE-SIG B). For example, from the HE-STF (or the HE-SIG B),256 FFT may be used in a 20 MHz channel, 512 FFT may be used in a 40 MHzchannel, and 1024 FFT may be used in an 80 MHz channel.

Information transmitted from each field included in the PPDU is the sameas that of an illustration of FIG. 20, and therefore a descriptionthereof is omitted.

The HE-SIG-A field is duplicated in a unit of 20 MHz and transmitted.

The HE-SIG B field may notify frequency bandwidth information allocatedon each STA basis and/or stream information in a corresponding frequencyband. The HE-SIG-B field includes information about each STA andinformation about each STA can be included for each HE-SIG-B field of a20 MHz unit. In this case, FIG. 28 illustrates a case in which 20 MHz isassigned on each STA basis, but for example, when 40 MHz is assigned tothe STA, the HE-SIG-B field may be duplicated and transmitted in a unitof 20 MHz.

In a situation of supporting different bandwidths on each BSS basis,when allocating some bandwidths having a small interference level froman adjacent BSS to the STA, as described above, it may be preferable notto transmit the HE-SIG-B field over an entire band.

In FIGS. 20 to 22, the data field is a payload and may include a servicefield, a scrambled PSDU, tail bits, and padding bits.

A HE format PPDU of FIGS. 20 to 22 may be classified through a RepeatedL-SIG (RL-SIG) field, which is a repetition symbol of an L-SIG field.The RL-SIG field is inserted in front of the HE SIG-A field, and eachSTA may classify a format of the PPDU received using the RL-SIG fieldinto the HE format PPDU.

A method in which an AP operating in a WLAN system transmits data to aplurality of STAs on the same time resource may be referred to asdownlink multi-user (DL MU) transmission. In contrast, a method in whicha plurality of STAs operating in a WLAN system transmits data to an APon the same time resource may be referred to as uplink multi-usertransmission.

Such DL MU transmission or UL MU transmission may be multiplexed on afrequency domain or a spatial domain.

When multiplexed on a frequency domain, different frequency resources(e.g., subcarrier or tone) may be allocated as a downlink or uplinkresource to each of a plurality of STAs based on orthogonal frequencydivision multiplexing (OFDMA). A transmitting method through differentfrequency resources in the same time resource may be referred to as‘DL/UL OFDMA transmission’.

When multiplexed on a spatial domain, different space stream may beallocated as a downlink or uplink resource to each of a plurality ofSTAs. A transmitting method through different spatial stream in the sametime resource may be referred to as ‘DL/UL MU MIMO’.

Hereinafter, a multi-user uplink transmitting method in a WLAN systemwill be described.

A current WLAN system does not support UL MU transmission due to thefollowing restrictions.

The current WLAN system does not support synchronization of transmittingtiming of uplink data transmitted from a plurality of STAs. For example,in an existing WLAN system, when it is assumed that a plurality of STAstransmit uplink data through the same time resource, in the current WLANsystem, a plurality of STAs each may not know transmitting timing ofuplink data of other STAs. Therefore, the AP cannot receive uplink datafrom each of a plurality of STAs on the same time resource.

Further, in the current WLAN system, frequency resources used fortransmitting uplink data by a plurality of STAs may be overlapped. Forexample, when oscillators of each of a plurality of STAs are different,frequency offset may be differently represented. When a plurality ofSTAs each having different frequency offset simultaneously transmit anuplink through different frequency resources, a portion of a frequencydomain used by each of a plurality of STAs may be overlapped.

Further, in an existing WLAN system, a power control for each of aplurality of STAs is not performed. The AP dependent on a channelenvironment and a distance between each of the plurality of STAs and theAP may receive a signal of different power from each of the plurality ofSTAs. In such a case, a signal arriving with weak power may berelatively difficultly detected by the AP, compared with a signalarriving with strong power.

Accordingly, the present invention provides a UL MU transmitting methodin a WLAN system.

FIG. 23 is a diagram illustrating an uplink multi-user transmittingprocedure according to an embodiment of the present invention.

Referring to FIG. 23, the AP instructs to prepare UL MU transmission toSTAs participating in UL MU transmission, receives a UL MU data framefrom corresponding STAs, and transmits an ACK frame (Block Ack (BA)frame) in response to the UL MU data frame.

First, by transmitting a UL MU trigger frame 2310, the AP instructs toprepare UL MU transmission to STAs to transmit UL MU data. Here, an ULMU scheduling frame may be referred to as a term ‘UL MU schedulingframe’.

Here, the UL MU trigger frame 2310 may include control information suchas STA identifier (ID)/address information, resource allocationinformation to be used by each STA, and duration information.

The STA ID/address information means information about an identifier oran address for specifying each STA that transmits uplink data.

The resource allocation information means information about an uplinktransmitting resource (e.g., frequency/subcarrier information assignedto each STA in UL OFDMA transmission, stream index assigned to each STAin UL MU MIMO transmission) assigned on each STA basis.

The duration information means information for determining a timeresource for transmission of an uplink data frame transmitted by each ofa plurality of STAs.

For example, the duration information may include interval informationof a Transmit Opportunity (TXOP) allocated for uplink transmission ofeach STA or information (e.g., bit or symbol) about an uplink framelength.

Further, the UL MU trigger frame 2310 may further include controlinformation such as MCS information and coding information that shoulduse upon transmitting an UL MU data frame on each STA basis.

The control information may be transmitted from a HE-part (e.g., aHE-SIG-A field or a HE-SIG-B field) of the PPDU that carries the UL MUtrigger frame 2310 or a control field (e.g., a Frame Control field ofthe MAC frame) of the UL MU trigger frame 2310.

The PPDU that carries the UL MU trigger frame 2310 has a structurestarting from an L-part (e.g., L-STF field, L-LTF field, and L-SIGfield). Accordingly, legacy STAs may set a Network Allocation Vector(NAV) from the L-SIG field through L-SIG protection. For example, legacySTAs may calculate an interval (hereinafter, ‘L-SIG guard interval’) forNAV setting based on information about a data length and a data rate inthe L-SIG. The legacy STAs may determine that there is no data to betransmitted thereto for the calculated L-SIG guard interval.

For example, the L-SIG guard interval may be determined with the sum ofa MAC duration field value of the UL MU trigger frame 2310 and theremaining intervals after the L-SIG field of the PPDU that carries theUL MU trigger frame 2310. Accordingly, the L-SIG guard interval may beset to a value to an interval that transmits the ACK frame 2330 (or BAframe) transmitted to each STA according to a MAC duration value of theUL MU trigger frame 2310.

Hereinafter, a method of allocating a resource for UL MU transmission toeach STA will be described in detail. For convenience of description, afield including control information is divided, but the presentinvention is not limited thereto.

The first field may classify and indicate UL OFDMA transmission and ULMU MIMO transmission. For example, when the first field is ‘0’, thefirst field may indicate UL OFDMA transmission, and when the first fieldis ‘1’, the first field may indicate UL MU MIMO transmission. A size ofthe first field may be 1 bit.

The second field (e.g., STA ID/address field) notifies an STA ID or STAaddress to participate in UL MU transmission. A size of the second fieldmay be configured with the bit number for notifying the STA ID×the STAnumber to participate in UL MU. For example, when the second field is 12bits, the second field may indicate an ID/address of each STA on 4 bitbasis.

For UL MU transmission, the third field (e.g., resource allocationfield) indicates a resource area applied to each STA. In this case, aresource area applied to each STA may be sequentially indicated to eachSTA in order of the second field.

When the first field value is ‘0’, the first field represents frequencyinformation (e.g., frequency index, subcarrier index) for UL MUtransmission in order of the STA ID/address included in the secondfield, and when the first field value is ‘1’, the first field representsMIMO information (e.g., stream index) for UL MU transmission in order ofthe STA ID/address included in the second field.

In this case, because the third field may notify several indexes (i.e.,frequency/subcarrier index or stream index) of one STA, a size of thethird field may be configured with a plurality of bits (or may beconfigured in a bitmap format)×the STA number to participate in UL MUtransmission.

For example, it is assumed that the second field is set in order of ‘STA1’ and ‘STA 2’ and that the third field is set in order of ‘2’ and ‘2’.

In this case, when the first field is ‘0’, in the STA 1, a frequencyresource is allocated from a superordinate (or subordinate) frequencydomain, and in the STA 2, next frequency resources may be sequentiallyallocated. For example, in a 80 MHz band, when OFDMA of a 20 MHz unit issupported, the STA 1 may use a superordinate (or subordinate) 40 MHzband, and the STA 2 may use a next 40 MHz band.

However, when the first field is ‘1’, in the STA 1, superordinate (orsubordinate) stream is allocated, and in the STA 2, next stream may besequentially allocated. In this case, a beamforming method according toeach stream may be previously designated or in a third field or a fourthfield, more detailed information about a beamforming method according tostream may be included.

Each STA transmits UL MU data frames 2321, 2322, and 2323 to the APbased on the UL MU trigger frame 2310 transmitted by the AP. Here, eachSTA may receive the UL MU trigger frame 2310 from the AP and transmit ULMU data frames 2321, 2322 and 2323 to the AP after SIFS.

The each STA may determine a specific frequency resource for UL OFDMAtransmission based on resource allocation information of the UL MUtrigger frame 2310 or spatial stream for UL MU MIMO transmission.

Specifically, in UL OFDMA transmission, each STA may transmit an uplinkdata frame on the same time resource through different frequencyresources.

Here, the STA 1 to the STA 3 each may receive allocation of differentfrequency resources for uplink data frame transmission based on STAID/address information and resource allocation information included inthe UL MU trigger frame 3110. For example, STA ID/address informationmay sequentially indicate the STA 1 to the STA 3, and resourceallocation information may sequentially indicate a frequency resource 1,a frequency resource 2, and a frequency resource 3. In this case, theSTA 1 to the STA 3 sequentially indicated based on STA ID/addressinformation may receive allocation of the frequency resource 1, thefrequency resource 2, and the frequency resource 3, respectively,sequentially indicated based on resource allocation information. Thatis, the STA 1 may transmit uplink data frames 2321, 2322, and 2323 tothe AP through the frequency resource 1, the STA 2 may transmit uplinkdata frames 2321, 2322, and 2323 to the AP through the frequencyresource 2, and the STA 3 may transmit uplink data frames 2321, 2322,and 2323 to the AP through the frequency resource 3.

Further, in UL MU MIMO transmission, each STA may transmit an uplinkdata frame on the same time resource through at least one differentstream of a plurality of spatial stream.

Here, the STA 1 to the STA 3 each may receive allocation of spatialstream for uplink data frame transmission based on STA ID/addressinformation and resource allocation information included in the UL MUtrigger frame 2310. For example, the STA ID/address information maysequentially indicate the STA 1 to the STA 3, and the resourceallocation information may sequentially indicate spatial stream 1,spatial stream 2, and spatial stream 3. In this case, the STA 1 to theSTA 3 sequentially indicated based on the STA ID/address information mayreceive allocation of spatial stream 1, spatial stream 2, and spatialstream 3, respectively, sequentially indicated based on resourceallocation information. That is, the STA 1 may transmit uplink dataframes 2321, 2322 and 2323 to the AP through spatial stream 1, the STA 2may transmit uplink data frames 2321, 2322 and 2323 to the AP throughspatial stream 2, and the STA 3 may transmit uplink data frames 2321,2322 and 2323 to the AP through spatial stream 3.

As described above, a transmission duration (or transmission terminationtime point) of uplink data frames 2321, 2322, and 2323 transmitted byeach STA may be determined by MAC duration information included in theUL MU trigger frame 2310. Therefore, each STA may synchronizetransmission termination time points of uplink data frames 2321, 2322,and 2323 (or an uplink PPDU that carries an uplink data frame) throughbit padding or fragmentation based on a MAC duration value included inthe UL MU trigger frame 2310.

The PPDU that carries the uplink data frames 2321, 2322, and 2323 may beformed in a new structure even without an L-part.

Further, in a case of UL MU MIMO transmission or in a case of UL OFDMAtransmission of a subband form of less than 20 MHz, an L-part of a PPDUthat carries uplink data frames 2321, 2322 and 2323 may be transmittedin a Single Frequency Network (SFN) form (i.e., an entire STAsimultaneously transmits the same L-part configuration and contents).However, in a case of UL OFDMA transmission of a subband form of 20 MHzor more, an L-part of a PPDU that carries the uplink data frames 2321,2322 and 2323 may be transmitted in a 20 MHz unit in a band in whicheach STA is allocated.

As described above, in the UL MU trigger frame 2310, a MAC durationvalue may be set to a value to an interval that transmits the ACK frame2330, and an L-SIG guard interval may be determined based on a MACduration value. Therefore, the legacy STA may set a NAV to the ACK frame3130 through an L-SIG field of the UL MU trigger frame 2310.

When an uplink data frame may be fully configured with information ofthe UL MU trigger frame 2310, a HE-SIG field (i.e., an area thattransmits control information about a data frame configuration method)within a PPDU that carries the UL MU trigger frame 2310 may not berequired. For example, the HE-SIG-A field and/or the HE-SIG-B field maynot be transmitted. Further, the HE-SIG-A field and the HE-SIG-C fieldmay be transmitted, and the HE-SIG-B field may not be transmitted.

The AP may transmit an ACK frame 2330 (or BA frame) in response touplink data frames 2321, 2322, and 2323 received from each STA. Here,the AP may receive uplink data frames 2321, 2322, and 2323 from each STAand transmit the ACK frame 2330 to each STA after SIFS.

When a structure of an existing ACK frame is equally used, an RA fieldhaving a size of 6 octets may include AID (or Partial AID) of STAsparticipating in UL MU transmission.

Alternatively, when an ACK frame of a new structure is configured, theACK frame may be configured in a form for DL SU transmission or DL MUtransmission. That is, in a case of DL SU transmission, the ACK frame2330 may be sequentially transmitted to each STA participating in UL MUtransmission, and in a case of DL MU transmission, the ACK frame 2330may be simultaneously transmitted to each STA participating in UL MUtransmission through a resource (i.e., frequency or stream) allocated toeach STA.

The AP may transmit only the ACK frame 2330 of a UL MU data framesucceeded in reception to a corresponding STA. Further, the AP maynotify with ACK or NACK whether reception is succeeded through the ACKframe 2330. When the ACK frame 2330 includes NACK information, the ACKframe 2330 may include information (e.g., UL MU scheduling information)for a subsequent procedure or the reason of NACK.

Alternatively, the PPDU that carries the ACK frame 3130 may be formed ina new structure without an L-part.

The ACK frame 2330 may include STA ID or address information, but whenorder of STAs indicated in the UL MU trigger frame 2310 is equallyapplied, the ACK frame 2330 may omit STA ID or address information.

Further, a frame for next UL MU scheduling by extending a TXOP (i.e.,L-SIG guard interval) of the ACK frame 2330 and a control frameincluding adjustment information for next UL MU transmission may beincluded within the TXOP.

For UL MU transmission, an adjustment process of correspondingsynchronization between STAs before or within a procedure of FIG. 23 maybe added.

Hereinafter, the present invention provides a method of configuring aframe structure including both single user (SU) transmission andmulti-user (MU) transmission.

In the present invention, MU transmission includes an entire case inwhich multi-users simultaneously transmit in the same time domain, as inOFDMA or MU MIMO.

Hereinafter, in a description of the present invention, a ‘frame’ maymean a DL/UL MAC frame (i.e., MAC control frame, MAC management frame,or data frame) self and may mean a DL/UL (SU/MU) PPDU that carries theDL/UL MAC frame.

Definition of Each Mode

1) DL SU and DL MU: in a downlink, i.e., when the AP transmits a signalto STAs, the difference between the SU and the MU represents whether theAP allocates an entire band (e.g., bandwidth of the PPDU) to one STA orseveral STAs.

However, in the DL, the AP contends and transmits a channel regardlessof the SU or the MU, and because a limitation problem of power of the APis less than that of the STA, the AP does not require a separatedistinction. Further, in an OFDMA structure, even if the SU is used, theAP generally allocates an entire band to one STA.

2) UL SU: in an uplink, i.e., when the STA transmits a signal to the AP,the STA secures and transmits a medium through direct channel contendingwithout a trigger frame of the AP. Hereinafter, in a description of thepresent invention, when a trigger frame exists, a case in which only oneSTA transmits an uplink data frame is referred to as an UL MU.

3) UL MU: in an uplink, i.e., when the STA transmits a signal to the AP,the AP previously transmits a DL frame (e.g., trigger frame) and the STAsecures a channel to transmit a UL data frame and transmits an ULsignal. That is, in a channel that is not occupied by the DL frame(e.g., trigger frame), an uplink resource is not allocated.

For example, the AP may transmit a trigger frame, and the STA maytransmit an UL frame according to the indication. In this case, asdescribed above, even if one STA transmits an UL frame, in a channelsecured by the DL frame, when the STA transmits the UL frame, it isreferred to as an UL MU. That is, when a trigger frame is transmitted,even if only one STA transmits an UL data frame, it is an UL MU.

Hereinafter, when describing the present invention, a HE-SIG1 field maybe referred to as an HE-SIG-A field, and a HE-SIG2 field may be referredto as an HE-SIG-B field.

FIG. 24 is a diagram illustrating UL MU transmission according to anembodiment of the present invention.

In FIG. 24, when the AP transmits a trigger frame, MU STAs transmit ULdata. However, any STA within the BSS may not recognize that an UL MUframe exists.

In FIG. 24, other STA1 receives a trigger frame within the BSS, but doesnot receive an UL MU frame. Therefore, other STA1 may receive thetrigger frame and transmit UL data to the AP after EIFS(EIFS=aSIFSTime+DIFS+EstimatedACKTxTime). In a legacy 801.11 system, theAP may receive the UL MU frame and complete transmission of an ACK framewithin EIFS. However, because a length of an UL MU packet of a 11axsystem may be longer than that of a legacy system, an UL frame in whichother STA1 transmits after EIFS may collide with UL MU datacommunication by the trigger frame.

FIG. 25 is a diagram illustrating UL MU transmission according to anembodiment of the present invention.

In FIG. 25, the AP transmits a trigger frame, and MU STAs transmit ULdata. However, any STA of OBSS may not recognize existence of a triggerframe and an ACK frame.

In FIG. 25, other STA2 does not overhear a trigger frame. Other STA2 mayoverhear only an UL MU frame. Therefore, other STA2 may transmit apacket thereof after EIFS from termination of the UL MU frame. However,because a length of the DL MU ACK frame may be longer than that of alegacy ACK/BA frame, the DL MU ACK frame may collide with a frame inwhich other STA2 transmits and an ACK frame in which MU STAs transmit.

In an UL MU procedure of FIGS. 24 and 25, additional TXOP protection forpreventing collision with transmitting data of other STAs is required.Hereinafter, TXOP protection for such a UL MU procedure will bedescribed.

A TXOP means a transmission opportunity. In other words, the TXOPrepresents an interval of time when a particular Quality of Service(QoS) STA has the right to initiate frame exchange sequence onto thewireless medium (WM). The TXOP may be defined to a start time and amaximum duration. The TXOP may be acquired by successful channelcontending of the STA or may be allocated by a Hybrid Coordinator (HC).The TXOP protection means interference prevention of other STAs for aframe transmitting and receiving interval of the STA for protection ofsuch TXOP. For example, by NAV setting of other STAs other than an STA,which is a transmitting and receiving target at an interval oftransmitting and receiving data thereof, a specific STA may protect aTXOP thereof. Hereinafter, a length of a TXOP related frame may mean alength of a time domain.

For TXOP protection, the above-described L-SIG TXOP protection may beapplied. The L-SIG TXOP protection is used for representing a PPDUlength or is used for representing a TXOP duration in a 802.11n system.An L-SIG length value of the trigger frame and/or the UL MU frame may beused for setting a TXOP length. When a length field value of L-SIG ofthe trigger frame and/or the UL MU frame is set to a TXOP length, both alegacy STA and 11ax STA may analyze the duration to a PPDU length or aTXOP length and set a NAV by a length field value of the L-SIG.

However, when using L-SIG TXOP protection, the legacy STA may recognizean L-SIG length to a PPDU length instead of setting a NAV with an L-SIGlength. Therefore, when data are not detected for a PPDU lengthaccording to implementation of a transmitting and receiving device, thelegacy STA may be changed to an idle state to attempt again mediumaccess. Further, because a resource of the L-SIG is very limited, thereis a problem that a value that can signal is limited. Because the L-SIGincludes a parity bit of only 1 bit, there is a problem that reliabilityof data is deteriorated. Finally, outdoor STAs may not well decode theL-SIG. Hereinafter, a TXOP protection method that does not use lengthinformation included in the L-SIG field will be described.

TXOP protection may be performed using a trigger frame. The triggerframe may use a legacy PPDU format or a 11ax PPDU format.

When the trigger frame is formed in a legacy PPDU format, a legacy STA,having received the trigger frame may set a TXOP using a duration fieldincluded in a MAC header. When the trigger frame is formed in a 11axPPDU format, a legacy STA may not set a TXOP.

When a trigger frame is a legacy PPDU, an 11ax STA, having received thetrigger frame may set a TXOP using a duration field included in a MACheader. When the trigger frame is formed in a 11ax PPDU format, the 11axSTA may set a TXOP using a duration field included in a MAC header.However, when a SIG B field of the trigger frame includes STA specificinformation, other STAs may not decode a MAC header. Particularly, inFIG. 25, when an STA of the OBSS decodes headers up to a MAC header of asignal frame of other BSSs to set a TXOP, system efficiency may bedeteriorated. Therefore, the present invention provides a method ofsetting a TXOP using an HE-SIG field.

The present invention provides a TXOP protection method of an UL MUprocedure using a length field. In this specification, a length may bereferred to as a duration. Length/duration information may berepresented with a micro second (μs) unit or a symbol unit and may besignaled in a bit or octet (byte) unit. In an embodiment of the presentinvention, positions of the HE-SIG-A field and the HE-SIG-B field may bechanged, and a HE-SIG-C field may be added. For convenience,HE-SIG-A/B/C fields have been divided and described, but the entire orsome of a description on the HE-SIG-A field may be applied to theHE-SIG-B field. When two HE-SIG-B fields exist in a continuous form, thetwo HE-SIG-B fields may be referred to as a HE-SIG-B field and aHE-SIG-C field.

FIG. 26 is a diagram illustrating a HE frame according to an embodimentof the present invention.

In FIG. 26, a HE PPDU includes legacy preambles L-STF, L-LTF, and L-SIG,HE preambles HE-SIG-A, HE-SIG-B, HE-STF, and HE-LTF, and a payload. InFIG. 26, the PPDU is a payload and includes a MPDU, and the MPDU furtherincludes a MAC header, a payload, and a Frame Check Sequence (FCS).

The L-SIG field may include a length field. When a length field of theL-SIG field is used for TXOP protection, the length field may be usedfor NAV setting of a TXOP interval 1 including a frame followingimmediately after a current frame. In FIG. 26, a following frametransmitted and received immediately after a current frame isrepresented with an ACK frame. However, a following frame transmittedand received immediately after a current frame may be an ACK frame or aBA frame. Further, when a current frame is an RTS frame, a subsequentframe may be a CTS frame. That is, a length field may indicate aduration 1 that completes a transmitting and receiving procedureincluding a current frame. When a length field of the L-SIG field is notused for TXOP protection, the length field may indicate a length of acurrent frame.

The HE-SIG-A field may include an HE-SIG-B length field indicating alength 2 of the HE-SIG-B field.

The HE-SIG-A field may include a length field indicating a length 3 of acurrent frame. The length field may be included in the HE-SIG-B field.When a length field of the L-SIG field is not used for TXOP protection,a length field of the HE-SIG field may be omitted.

A length field may be included in the MAC header. The length field ofthe MAC header may be used for TXOP protection. The length field of theMAC header indicates a length 4 including a length of the remainingframes and a length of a continuously transmitted and received frame andmay be used for NAV setting.

FIG. 27 is a diagram illustrating a UL MU procedure and TXOP protectionaccording to an embodiment of the present invention.

FIG. 27 illustrates an UL MU procedure in which the AP STA transmits atrigger frame and in which a plurality of STAs transmit an UL MU frameaccording to the trigger frame and in which the AP STA transmits an ACKframe. FIG. 27 illustrates an operation on one STA of a plurality ofSTAs that transmit an UL MU frame. FIG. 27 illustrates an embodiment inwhich a trigger frame is transmitted as a PHY structure, but a frame ofFIG. 27 may be a MAC frame structure including a MPDU.

The L-SIG field of the trigger frame may include a length field. Whenthe length field of the L-SIG field is used for TXOP protection, thelength field may be used for NAV setting of a length 1 including an ULMU procedure.

The HE-SIG-A field of the trigger frame may include a HE-SIG-B lengthfield indicating a length 2 of the HE-SIG-B field. The HE-SIG-B field ofthe trigger frame includes information about UL MU transmission. In anembodiment, the HE-SIG-B field is used for a DL, a HE-SIG-C field may beadditionally included in the HE-SIG-B field, and in such a case, theHE-SIG-B field may include a length field of the HE-SIG-C field.

The HE-SIG-A field of the trigger frame may include a field indicating alength 3 for TXOP protection. The length field for TXOP protection maybe referred to as a TXOP protection field or a TXOP duration (interval)field. The TXOP duration field may indicate the remaining time/intervallength in a frame exchange procedure of the STA. A TXOP interval inwhich the TXOP duration field represents may include a length ofsubsequent frames. As shown in FIG. 27, the TXOP interval may include alength of a UL MU frame and a length of an ACK frame. In anotherembodiment, the TXOP interval may further include the remaining signalframe portions after a signal field including a TXOP duration field. TheTXOP duration field may be used by 11ax STA of an outdoor/weak channelfailed in decoding of the L-SIG field. However, the TXOP duration fieldmay be used by entire 11 ax STAs. A TXOP duration in which the TXOPduration field represents may represent a length from the end of theHE-SIG-A field to the end of the ACK frame. The TXOP duration of thetrigger frame may further include an Inter Frame Space (IFS) timebetween the trigger frame and the UL MU frame and an IFS time betweenthe UL MU frame and the ACK frame.

The trigger frame may be transmitted in a MAC structure, and in thiscase, analysis of a length field of the 11ax frame may be used. The TXOPprotection interval is an entire UL MU procedure including an intervalof the UL MU frame and the ACK frame. In this case, the L-SIG lengthfield may indicate a current frame length, and a length field of the MACheader may indicate a TXOP length.

FIG. 28 is a diagram illustrating a UL MU procedure and TXOP protectionaccording to an embodiment of the present invention and particularlyillustrates an example of a cascade structure in which a UL MU procedureis transmitted together with a DL MU frame.

In FIG. 28, a description on fields of lengths (1), (2), (3), and (5) isthe same as that of FIGS. 26 and 27. However, in an embodiment of FIG.28, two SIG-B fields are included. Two HE-SIG-B fields may be twoseparately coded HS-SIG-B fields. That is, two HE-SIG-B fields each mayinclude information of the DL and information of the UL. In anembodiment, one HE-SIG-B field including information of the DL andinformation of the UL may be included. In such a case, because theHE-SIG-B field is extended, a performance may be deteriorated.Hereinafter, an embodiment including two HE-SIG-B fields will bedescribed.

When a frame includes two HE-SIG-B fields, an indicator is required inwhich a general 11ax frame includes one HE-SIG-B field and in which acascade frame includes two HE-SIG-B fields. Even in a case in which aframe includes one HE-SIG-B field, an indicator may be required in whichone HE-SIG-B field represents DL information or UL information.Additionally, an indicator indicating whether the frame is a MAC frameor a PHY frame may be required.

Therefore, a frame according to an embodiment of the present inventionmay add a 1 bit indicator to the HE-SIG-B to indicate whether the frameis a general 11ax MAC frame. Even in a case in which a frame has acascade format, a HE-SIG-B field for trigger information may be alwaysincluded in front of the 11ax MAC frame. The STA, having received such aframe may process a receiving frame according to an indication bit valueof the HE-SIG-B field.

In an embodiment, when a value of an indicator 1 bit of the HE-SIG-Bfield is 0x0, the receiving STA recognizes and decodes a frame as aframe of FIG. 26, and when a value of 1 bit is 0x1, the receiving STArecognizes the HE-SIG-B field as trigger information to acquireinformation for UL transmission and determine whether an additionalHE-SIG-B field exists. The HE-SIG-B field analyzed as triggerinformation may include a length field of the additional HE-SIG-B field.When a value of such a length field is 0, the receiving STA mayrecognize a corresponding frame as a trigger PHY frame having noadditional HE-SIG-B. That is, when a length value of the receiving STAis 0, a frame may be analyzed, as in an embodiment described withreference to FIG. 27, and when a length value of the receiving STA is 1,a frame may be analyzed, as in an embodiment described with reference toFIG. 28.

In an embodiment, two or more HE-SIG-B fields may exist. For example,three HE-SIG-B fields may be used for the DL, for an UL bufferstate/channel state report, and for UL data transmission. In order toindicate continued UL transmission, an ACK/BA frame of the APtransmitted after receiving the UL frame may perform a function of apolling frame indicating next UL transmission. In such a case, anACK/BA+polling frame may be transmitted instead of the ACK/BA frame. Inthis case, contents constituting the HE-SIG-B field may be changedaccording to each object. For example, when length information of (3) istransmitted to the HE-SIG-B field, a length field of (3) may not beincluded in the HE-SIG-B field used for UL control and lengthinformation of (3) may be included in the HE-SIG-B field used fortransmitting DL information.

In another embodiment, the HE-SIG-A field may signal the entire of thenumber of HE-SIG-B fields, MCS information, and length information.Alternatively, the HE-SIG-A field represents MCS information and lengthinformation of a first preceding HE-SIG-B field, and a precedingHE-SIG-B field may represent MCS information and length information ofnext HE-SIG-B field. In an embodiment, order of HE-SIG-B fields mayinclude information about a preceding HE-SIG-B field having a morerobust MCS and a following HE-SIG-B field.

FIG. 29 is a diagram illustrating a UL MU procedure and TXOP protectionaccording to an embodiment of the present invention and illustrates anexample of a cascade structure in which a UL MU procedure is togethertransmitted with a DL MU frame. Particularly, FIG. 29 illustrates anembodiment of a cascade structure when trigger information istransmitted to the MAC frame.

As shown in FIG. 29, the DL frame includes one MAC header+payload+FCS(MPDU) that carries trigger information. A MPDU including triggerinformation may be one MPDU of an A-MPDU or may be one MPDU of a MU MIMOand MU OFDMA.

Even in an embodiment of FIG. 29, a TXOP duration field may be includedin an HE-SIG-A field. However, information in which a MAC framecorresponding to trigger information or trigger information is includedin the PPDU should be indicated. For example, information may indicateto include broadcast AID in the HE-SIG-B field and to enable a receivingSTA to read an entire MAC header. Alternatively, by fixing a position ofan MPDU that loads trigger information, the receiving STA may determineonly a corresponding position to determine whether trigger informationis included.

FIG. 30 is a diagram illustrating a UL MU frame and TXOP protectionaccording to an embodiment of the present invention.

The UL MU frame at the left side of FIG. 30 represents a UL MU framestructure of an STA that transmits data to an area of a superordinate 10MHz in a 20 MHz band. In an UL frame structure of FIG. 30, a HE-SIG-Bfield and a HE-SIG-C field may be omitted.

In an embodiment, a length field included in the L-SIG field mayrepresent a TXOP length 1 from the UL frame to a transmitted andreceived ACK frame.

The HE-SIG-A field may include a length field representing a length 2-1of a HE-SIG-B field. When the HE-SIG-C field exists, the HE-SIG-C fieldmay represent a length 2-2 of the HE-SIG-C field. When the frameincludes both the HE-SIG-B field and the HE-SIG-C field, the HE-SIG-Afield may signal a length 2-1 of the HE-SIG-B field, and the HE-SIG-Bfield may signal a length 2-2 of the HE-SIG-C field.

A length field of the HE-SIG-A field may represent a length 3 of acurrent frame (PPDU). However, in an embodiment, a length of the UL MUframe may be already indicated in the trigger frame. In such a case, alength field of the HE-SIG-A field is a TXOP duration field and mayindicate a length of an entire TXOP 4. Thereby, other STAs that do notrecognize the trigger frame may decode a TXOP duration field included inthe HE-SIG-A field to set NAV, and thus a TXOP of an UL MU datatransmitting STA may be protected.

The TXOP duration field has been described with reference to FIG. 27. ATXOP interval of the UL MU frame, i.e., the remaining time/intervallength of a frame exchange procedure may include a length of asubsequent frame, i.e., an ACK frame length. That is, a TXOP intervalfield of the UL MU frame may indicate a TXOP length including an ACKframe. As shown, the TXOP interval field of the UL MU frame may indicatea TXOP interval further including an IFS time between the UL MU frameand the ACK frame. In another embodiment, a TXOP interval indicated by aTXOP interval field included in the UL MU frame may further include theremaining signal frame portions after a signal field including the TXOPduration field.

Length information of the MAC header may indicate a TXOP length. Whentransmitting an MU frame, it is unnecessary that other STAs decodeheaders up to a MAC header of the MU frame. Therefore, lengthinformation of the MAC header may be used for other usage instead ofusing for NAV setting. For example, length information of the MAC headermay indicate a length of actual data that subtract padding.

Because a subsequent ACK frame transmits MU AKC information, unlike alegacy ACK/BA frame, a length thereof may be extended. Therefore, thesubsequent ACK frame should set a TXOP duration field such that a legacySTA and an 11ax STA set a NAV. A TXOP duration value in which a lengthfield of the ACK frame, which is a final frame of the UL MU procedurerepresents is the same as the remaining lengths of both a 11ax frame anda legacy frame. Therefore, the TXOP duration field may be set toindicate the L-SIG length, the HE-SIG-A length, and a length to a finalframe of the MAC header. In an embodiment, the TXOP duration of the ACKframe may be set to 0.

FIG. 31 is a diagram illustrating an STA apparatus according to anembodiment of the present invention.

In FIG. 31, the STA apparatus may include a memory 31010, a processor31020, and a radio frequency (RF) unit 31030. As described above, theSTA apparatus is a HE STA apparatus and may be an AP or a non-AP STA.

The RF unit 31030 is connected to the processor 31020 totransmit/receive a radio signal. The RF unit 31030 may up-convert datareceived from the processor in a transmitting and receiving band totransmit a signal.

The processor 31020 is connected to the RF unit 31030 to implement aphysical layer and/or a MAC layer according to the IEEE 802.11 system.The processor 31020 may be configured to perform operations according tothe various embodiments of the present invention shown in FIGS. 1 to 31.Furthermore, a module for implementing the operation of the STAaccording to the various embodiments of the present invention may bestored at the memory 31010 to be executed by the processor 31020.

The memory 31010 is connected to the processor 31020 to store variousinformation for driving the processor 31020. The memory 31010 may beincluded in the processor 31020 or disposed outside the processor 31020and may be connected to the processor 31020 by known means.

Furthermore, the STA apparatus may include a single antenna or multipleantennas. A detailed configuration of the STA apparatus of FIG. 31 maybe implemented so that contents described in the various embodiments ofthe present invention are independently applied or so that two or moreembodiments are simultaneously applied.

A method of transmitting and receiving data including receivingoperating mode information of the STA apparatus of FIG. 31 will be againdescribed with reference to the following flowchart.

FIG. 32 is a diagram illustrating a method of transmitting/receiving aUL MU according to an embodiment of the present invention.

FIG. 32 illustrates communication with a plurality of MU STAs thatperform UL MU transmission with the AP STA. However, an operation of oneSTA of a plurality of MU STAs will be described.

In FIG. 32, the MU STAs receive a trigger frame from the AP STA(S32010). In other words, the AP STA transmits the trigger frame to theMU STAs (S32010). The trigger frame may include resource unit allocationinformation for OFDMA transmission. The resource unit may allocate, forexample 26 tones to a most small unit.

The MU STAs may transmit an UL MU PPDU to the AP STA based on thetrigger frame (S32020). In other words, the AP STA may receive an UL MUPPDU based on the trigger frame (S32020).

The MU STAs may receive an ACK frame of the UL MU PPDU (S32030). Inother words, the AP STA may transmit an ACK frame of the received UL MUPPDU (S32030).

As shown in FIG. 27, the trigger frame may include a legacy preamble anda HE preamble, and the HE preamble may include a HE-SIG-A field and aHE-SIG-B field. The legacy preamble may include an L-STF, an L-LTF, andan L-SIG field.

The HE-SIG-A field of the trigger frame may include TXOP durationinformation/field representing a TXOP duration. The TXOP duration is atime interval for a frame exchange sequence of the STA. The frameexchange sequence of the STA may represent transmission and reception ofa trigger frame, an UL MU PPDU, and an ACK frame.

The UL MU PPDU includes a legacy preamble and a HE preamble, and the HEpreamble may include a HE-SIG-A field. The HE-SIG-A field of the UL MUPPDU may include TXOP duration information/field. A definition and usageof the TXOP duration is the same as those of a TXOP duration of thetrigger frame. A TXOP duration of the UL MU PPDU is a TXOP length of theremaining duration after the UL MU PPDU and includes an ACK framelength. This is because a target of TXOP that should be protected in theSTA that transmits the UL MU PPDU is an ACK frame transmitting intervalafter the UL MU PPDU.

The TXOP duration field of the trigger frame may represent a length intime from the trigger frame to the end of the ACK frame. Therefore, theTXOP duration of the trigger frame may include an IFS time betweenreception of the trigger frame and transmission of the UL MU PPDU, alength of the UL MU PPDU, and an ACK frame length and an IFS timebetween the UL MU PPDU transmission and ACK frame reception.

In another embodiment, the TXOP duration field of the trigger frame mayrepresent a length in time from the end of the HE-SIG-A field of thetrigger frame to the end of the ACK frame. Therefore, the TXOP durationof the trigger frame may include a length of the remaining portionsafter the HE-SIG-A field of the trigger frame, an IFS time betweentrigger frame reception and UL MU PPDU transmission, a length of the ULMU PPDU, and an ACK frame length and an IFS time between UL MU PPDUtransmission and ACK frame reception.

The TXOP duration field of the UL MU PPDU frame may represent a lengthin time from the UL MU PPDU to the end of the ACK frame. Therefore, theTXOP duration of the UL MU PPDU may include an ACK frame length and anIFS time between UL MU PPDU transmission and ACK frame reception.

In another embodiment, the TXOP duration field of the UL MU PPDU framemay represent a length in time from the end of the HE-SIG-A field of theUL MU PPDU to the end of the ACK frame. Therefore, the TXOP duration ofthe UL MU PPDU may include a length of the remaining portions after theHE-SIG-A field of the UL MU PPDU and an ACK frame length and an IFS timebetween UL MU PPDU transmission and ACK frame reception.

The UL MU PPDU may include a HE-STF, a HE-LTF, and a data field. TheHE-STF, the HE-LTF, and the data field of the UL MU PPDU may betransmitted through a bandwidth of a resource unit allocated to acorresponding STA.

In the aforementioned embodiments, elements and characteristics of thepresent invention have been combined in predetermined forms. Each of theelements or characteristics may be considered to be optional unlessotherwise described explicitly. Each of the elements or characteristicsmay be implemented in such a way as to be not combined with otherelements or characteristics. Furthermore, some of the elements and/orthe characteristics may be combined to form an embodiment of the presentinvention. The order of operations described in connection with theembodiments of the present invention may be changed. Some of theelements or characteristics of an embodiment may be included in anotherembodiment or may be replaced with corresponding elements orcharacteristics of another embodiment. It is evident that an embodimentmay be constructed by combining claims having no explicit citationrelation in the claims or may be included as a new claim by amendmentsafter filing an application.

The embodiment of the present invention may be implemented by variousmeans, for example, hardware, firmware, software or a combinationthereof. In the case of implementations by hardware, the embodiment ofthe present invention may be implemented using one or moreapplication-specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, microcontrollers and/ormicroprocessors.

In the case of an implementation by firmware or software, the embodimentof the present invention may be implemented in the form of a module,procedure or function for performing the aforementioned functions oroperations. A software code may be stored at the memory to be driven bythe processor. The memory may be located inside or outside the processorand may exchange data with the processor through a variety of knownmeans.

It is evident to those skilled in the art that the present invention maybe materialized in other specific forms without departing from theessential characteristics of the present invention. Accordingly, thedetailed description should not be construed as being limitative fromall aspects, but should be construed as being illustrative. The scope ofthe present invention should be determined by reasonable analysis of theattached claims, and all changes within the equivalent range of thepresent invention are included in the scope of the present invention.

INDUSTRIAL APPLICABILITY

In a wireless communication system of the present invention, an examplein which an uplink single user or multi-user transmitting method isapplied to an IEEE 802.11 system has been described, but the wirelesscommunication system can be applied to various wireless communicationsystems in addition to the IEEE 802.11 system.

What is claimed is:
 1. An uplink (UL) multi-user (MU) transmittingmethod of a station (STA) in a wireless LAN (WLAN) system, the UL MUtransmitting method comprising: receiving a trigger frame comprisingresource unit allocation information for orthogonal frequency divisionmultiple access (OFDMA) transmission; transmitting a UL MU PhysicalProtocol Data Unit (PPDU) based on the trigger frame; and receiving anACK frame of the UL MU PPDU, wherein the UL MU PPDU comprises a firstlegacy preamble and a first High Efficiency (HE) preamble, and the firstHE preamble comprises a first HE-SIG-A field and a first HE-SIG-B field,and wherein the first HE-SIG-A field comprises first TransmissionOpportunity (TXOP) duration information related to a first TXOPduration, and the first TXOP duration is set to include a time requiredto transmit the UL MU PPDU and the ACK frame.
 2. The method of claim 1,wherein the UL MU PPDU comprises a second legacy preamble and a secondHE preamble, and the second HE preamble comprises a second HE-SIG-Afield, and wherein the second HE-SIG-A field comprises second TXOPduration information related to a second TXOP duration, and the secondTXOP duration is set to include a time required to transmit the UL MUPPDU and the ACK frame.
 3. The method of claim 1, wherein the first TXOPduration further comprises an Inter Frame Space (IFS) time between thetrigger frame and the UL MU frame and an IFS time between the UL MUframe and the ACK frame.
 4. The method of claim 2, wherein the secondTXOP duration further comprises an IFS time between the UL MU PPDU andthe ACK frame.
 5. The method of claim 2, wherein the second HE preambleof the UL MU PPDU further comprises a High Efficiency-Short TrainingField (HE-STF), and a HE-Long Training Field (HE-LTF), and wherein theHE-STF, the HE-LTF, and a data field of the UL MU PPDU are transmittedthrough a bandwidth of an allocated resource unit.
 6. A station (STA)apparatus of a wireless LAN (WLAN) system, the STA apparatus comprising:a Radio Frequency (RF) unit that transmits and receives a wirelesssignal; and a processor that controls the RF unit, wherein the STAapparatus receives a trigger frame comprising resource unit allocationinformation for orthogonal frequency division multiple access (OFDMA)transmission, transmits a UL MU Physical Protocol Data Unit (PPDU) basedon the trigger frame, and receives an ACK frame of the UL MU PPDU,wherein the UL MU PPDU comprises a first legacy preamble and a firstHigh Efficiency (HE) preamble, and the first HE preamble comprises afirst HE-SIG-A field and a first HE-SIG-B field, and wherein the firstHE-SIG-A field comprises first Transmission Opportunity (TXOP) durationinformation related to a first TXOP duration, and the first TXOPduration is set to include a time required to transmit the UL MU PPDUand the ACK frame.
 7. The STA apparatus of claim 6, wherein the UL MUPPDU comprises a second legacy preamble and a second HE preamble, andthe second HE preamble comprises a second HE-SIG-A field, and whereinthe second HE-SIG-A field comprises second TXOP duration informationrelated to a second TXOP duration, and the second TXOP duration is setto include a time required to transmit the UL MU PPDU and the ACK frame.8. The STA apparatus of claim 6, wherein the first TXOP duration furthercomprises an Inter Frame Space (IFS) time between the trigger frame andthe UL MU frame and an IFS time between the UL MU frame and the ACKframe.
 9. The STA apparatus of claim 7, wherein the second TXOP durationfurther comprises an IFS time between the UL MU PPDU and the ACK frame.10. The STA apparatus of claim 7, wherein the second HE preamble of theUL MU PPDU further comprises a High Efficiency-Short Training Field(HE-STF), a HE-Long Training Field (HE-LTF), and wherein the HE-STF, theHE-LTF, and a data field of the UL MU PPDU are transmitted through abandwidth of an allocated resource unit.
 11. An uplink (UL) multi-user(MU) receiving method of an Access Point (AP)-Station (STA) in awireless LAN (WLAN) system, the UL MU receiving method comprising:transmitting a trigger frame comprising resource unit allocationinformation for orthogonal frequency division multiple access (OFDMA)transmission; receiving a UL MU Physical Protocol Data Unit (PPDU) basedon the trigger frame; and transmitting an ACK frame of the UL MU PPDU,wherein the UL MU PPDU comprises a first legacy preamble and a firstHigh Efficiency (HE) preamble, and the first HE preamble comprises afirst HE-SIG-A field and a first HE-SIG-B field, and wherein the firstHE-SIG-A field comprises first Transmission Opportunity (TXOP) durationinformation related to a first TXOP duration, and the first TXOPduration is set to include a time required to transmit the UL MU PPDUand the ACK frame.